Chapter II

CRUSTACEA (_CONTINUED_): ENTOMOSTRACA—BRANCHIOPODA—PHYLLOPODA—CLADOCERA—
WATER-FLEAS

SUB-CLASS I.—ENTOMOSTRACA.

The Entomostraca are mostly small Crustacea in which the segmentation of the body behind the head is very variable, both in regard to the number of segments and the kind of differentiation exhibited by those segments and their appendages. An unpaired simple eye, known as the Nauplius eye from its universal presence in that larval form, often persists in the adult, and though lateral compound eyes may be present they are rarely borne on movable stalks. In the adult the excretory gland (“shell-gland”) opens on the second maxillary segment, but in the larval state or early stages of development a second antennary gland may also be present, which disappears in the adult. The liver usually points forwards, and is simple and saccular in structure, and the stomach is not complicated by the formation of a gastric mill. With the exception of most Cladocera and Ostracoda the young hatch out in the Nauplius state.

=Order I. Branchiopoda.=[14]

The Branchiopods are of small or moderate size, with flattened and lobate post-cephalic limbs, and with functional gnathobases. Median and lateral eyes are nearly always present. The labrum is large, and the second maxillae are small or absent in the adult.

Branchiopods are found in every part of the world; a few are marine, but the great majority are confined to inland lakes and ponds, or to slowly-moving streams. The fresh waters, from the smallest pools to the largest lakes, often swarm with them, as do those streams which flow so slowly that the creatures can obtain occasional shelter among vegetation along the sides and bottom without being swept away, while even rivers of considerable swiftness contain some Cladocera. Several Branchiopods are found in the brackish waters of estuaries, and some occur in lakes and pools so salt that no other Crustacea, and few other animals of any kind, can live in them. The great majority swim about with the back downwards, collecting food in the ventral groove between their post-oral limbs, and driving it forwards, towards the mouth, by movements of the gnathobases (p. 10). The food collected in this way consists largely of suspended organic mud, together with Diatoms and other Algae, and Infusoria; the larger kinds, however, are capable of gnawing objects of considerable size, _Apus_ being said to nibble the softer insect larvae, and even tadpoles. Many Cladocera (e.g. _Daphnia_, _Simocephalus_) may be seen to sink to the bottom of an aquarium, with the ventral surface downwards, and to collect mud, or even to devour the dead bodies of their fellows, while _Leptodora_ is said to feed upon living Copepods, which it catches by means of its antennae.

The Branchiopoda fall naturally into two Sub-orders, the PHYLLOPODA including a series of long-bodied forms, with at least ten pairs of post-cephalic limbs, and the CLADOCERA with shorter bodies and not more than six pairs of post-cephalic limbs.

=Sub-Order 1. Phyllopoda.=

The Phyllopoda include a series of genera which differ greatly in appearance, owing to differences in the development of the carapace, which are curiously correlated with differences in the position of the eyes. Except in these points, the three families which the sub-order contains are so much alike that they may conveniently be described together.

In the BRANCHIPODIDAE the carapace is practically absent, being represented only by the slight backward projection on each side of the head which contains the kidney (Fig. 2); the paired eyes are supported on mobile stalks, and project freely, one on either side of the head.

In the APODIDAE[15] the head is broad and depressed, the ventral side being nearly flat, the dorsal surface convex; the hinder margin of the head is indicated dorsally by a transverse cervical ridge, bounded by two grooves, behind which the carapace projects backwards as a great shield, covering at least half the body, but attached only to the back of the head. In _Lepidurus productus_ the head and carapace together form an oval expansion, deeply emarginate at the hinder, narrower end, the sides of the emargination being toothed. The carapace has a strong median keel. The kidneys project into the space between the folds of skin which form the carapace, and their coils can be seen on each side, the terminal part of each kidney-tube entering the head to open at the base of the second maxilla. In all Branchiopoda with a well-developed carapace the kidney is enclosed in it in this way, whence the older anatomists speak of it as the “shell-gland.”

FIG. 2.—_Chirocephalus diaphanus_, female, × 5, Sussex. _D.O_, Dorsal
organ; _H_, heart; _Ov_, ovary; _U_, uterus; _V_, external
generative opening.
]

Associated with the development of the carapace, in this and in the next family, is a remarkable condition of the lateral eyes, which are sessile on the dorsal surface of the head, and near the middle line, the median eye being slightly in front of them. During embryonic life a fold of skin grows over all three eyes, so that a chamber is formed over them, which communicates with the exterior by a small pore in front.

In the LIMNADIIDAE the body is laterally compressed, and the carapace is so large that at least the post-cephalic part of the body, and generally the head also, can be enclosed within it.

FIG. 3.—_Limnetis brachyura_, × 15. (After G. O. Sars.)
]

In _Limnetis_ (Fig. 3) the dorsal surface of the head is bent downwards and is much compressed, the carapace being attached to it only for a short distance near the dorsal middle line. The sides of the carapace are bent downwards, and their margins can be pulled together by a transverse adductor muscle, so that the whole structure forms an ovoid or spheroidal case, from which the head projects in front, while the rest of the body is entirely contained within it. When the adductor muscle is relaxed the edges of the carapace gape slightly, like the valves of a Lamellibranch shell, and food-particles are drawn through the opening thus formed into the ventral groove by the movements of the thoracic feet, locomotion being chiefly effected by the rowing action of the second antennae, as in the Cladocera, to which all the Limnadiidae present strong resemblances in their method of locomotion, in the condition of the carapace, and in the form of the telson.

In _Limnadia_ and _Estheria_ the carapace projects not only backwards from the point of attachment to the head, but also forwards, so that the head can be enclosed by it, together with the rest of the body.

In all these genera the carapace is flexible along the middle dorsal line; in _Estheria_ especially the softening of the dorsal cuticle goes so far that a definite hinge-line is formed, and this, together with the deposition of the lateral cuticle in lines concentrically arranged round a projecting umbo, gives the carapace a strong superficial likeness to a Lamellibranch shell, for which it is said to be frequently mistaken by collectors.

The eyes of the Limnadiidae are enclosed in a chamber formed by a growth of skin over them, as in Apodidae, but the pore by which this chamber communicates with the exterior is even more minute than in _Apus_. The paired eyes are so close together that they may touch (_Limnadia_, _Estheria_) or fuse (_Limnetis_); they are farther back than in the Apodidae, while the ventral curvature of the head causes the median eye to lie below them. In all these points the eyes of the Limnadiidae are intermediate between those of _Apus_ and those of the Cladocera.

=Dorsal Organ.=—A structure very characteristic of adult Phyllopods is the “dorsal organ” (Figs. 2, 5, _D.O_), whose function is in many cases obscure. It is always a patch of modified cephalic ectoderm, supplied by a nerve from the anterior ventral lobe of the brain on each side; but its characters, and apparent function, differ in different forms. In the Branchipodidae the dorsal organ is a circular patch, far forward on the surface of the head (Figs. 2, 5, _D.O_). Its cells are arranged in groups, which remind one of the retinulae in a compound eye; each cell contains a solid concretion, and the concretions of a group may be so placed as to look like a badly-formed rhabdom. Claus,[16] who first called attention to this structure in the Branchipodidae, regarded it as a sense-organ. In Apodidae the dorsal organ is an oval patch of columnar ectoderm, immediately behind the eyes; it is slightly raised above the surrounding skin, and is covered by a very delicate cuticle (with an opening to the exterior?), and below it is a mass of connective tissue permeated by blood; Bernard has suggested that it is an excretory organ.

Most Limnadiidae resemble the Cladocera in the possession of a “dorsal organ” quite distinct from the above; in _Limnetis_ and _Estheria_ it has the form of a small pit, lined by an apparently glandular ectoderm, and this is its condition in many Cladocera; in _Limnadia lenticularis_ it is a patch of glandular epithelium on a raised papilla. _Limnadia_ has been observed to anchor itself to foreign objects by pressing its dorsal organ against them, and many Cladocera do the same thing; _Sida crystallina_, for example, will remain for hours attached by its dorsal organ to a waterweed or to the side of an aquarium. Structures resembling a dorsal organ occur in the larvae of many other Crustacea, but the presence of this organ in the adult is confined to Branchiopods, and indeed in many Cladocera it disappears before maturity. It is certain that the sensory and adhesive types of dorsal organ are not homologous, especially as rudimentary sense-organs may exist on the head of Cladocera together with the adhesive organ.

The =telson= differs considerably in the different genera. In the Branchipodidae[17] the anus opens directly backwards; and the telson carries two flattened backwardly directed plates, one on each side of the anus, the margins of each plate being fringed with plumose setae. In _Artemia_ the anal plates are rarely as large as in _Branchipus_, and never have their margins completely fringed with setae; in _A. salina_ from Western Europe, and in _A. fertilis_ (Fig. 4, A) from the Great Salt Lake of Utah, there is a variable number of setae round the apical half of each lobe, but in specimens of _A. salina_ from Western Siberia the number of setae may be very small, or they may be absent; in the closely allied _A. urmiana_ from Persia the anal lobes are well developed in the male, each lobe bearing a single terminal hair, but they are altogether absent in the female. Schmankewitch and Bateson have shown that there is a certain relation between the salinity of the water in which _Artemia salina_ occurs and the condition of the anal lobes, specimens from denser waters having on the whole fewer setae; the relation is, however, evidently very complex, and further evidence is wanted before any more definite statements can be made.

FIG. 4.—=A=, Ventral view of the anal region in _Artemia fertilis_,
from the Great Salt Lake; =B=, ventral view of the telson and
neighbouring parts of _Lepidurus productus_; =C=, side view of the
telson and left anal lobe of _Estheria_ (sp.?).
]

In the Apodidae the anal lobes have the form of two-jointed cirri, often of considerable length; in _Apus_ the anus is terminal, but in _Lepidurus_ (Fig. 4, B) the dorsal part of the telson is prolonged backwards, so as to form a plate, on the ventral face of which the anus opens, much as in the Malacostraca.

In the Limnadiidae (Fig. 4, C) the telson is laterally compressed and produced, on each side of the anus, into a flattened, upwardly curved process, sharply pointed posteriorly, and often serrate; the anal lobes are represented by two stout curved spines, while in place of the dorsal prolongation of _Lepidurus_ we find two long plumose setae above the anus. In the characters of the telson and anal lobes, as in those of the head, the Limnadiidae approximate to the Cladocera. In _Limnetis brachyura_ the ventral face of the telson is produced into a plate projecting backwards below the anus, in a manner which has no exact parallel among other Crustacea.

The =appendages= of the Phyllopoda are fairly uniform in character, except those affected by the sexual dimorphism, which is usually great.

FIG. 5.—_Chirocephalus diaphanus_, male. Side view of head, showing
the large second antenna, _A_{2}_, with its appendage _Ap_, above
which is seen the filiform first antenna; _D.O_, dorsal organ;
_E_{1}_, median eye.
]

FIG. 6.—_Chirocephalus diaphanus._ Second antenna of male, uncoiled.
]

Of the cephalic appendages, the first antennae are generally small, and are never biramous; in _Branchipus_ and its allies they are simple unjointed rods, in some species of _Artemia_ they are three-jointed, in _Apus_ they are feebly divided into two joints, while in _Estheria_ they are many-jointed. The second antennae are the principal organs of locomotion in the Limnadiidae, where they are large and biramous; in all other Phyllopoda they are uniramous in the female, being either unjointed triangular plates as in _Chirocephalus_ (Fig. 2), or minute vestigial filaments as in _Apus_, in which genus Zaddach, Huxley, and Claus have all failed to find any trace of a second antenna in some females. In the male Branchipodidae the second antennae are modified to form claspers, by which the female is seized, the various degrees of complication which these claspers exhibit affording convenient generic characters. In _Branchinecta_ each second antenna is a thick, three-jointed rod, the last joint forming a claw, while the second joint is serrate on its inner margin; in _Branchipus_ the base is much thickened, and bears on its inner side a large filament (perhaps represented by the proximal tubercle of _Branchinecta_ and _Artemia_), which looks like an extra antenna. In _Streptocephalus_ the terminal joint of the antenna is bifid, and there is a basal filament like that of _Branchipus_; in _Chirocephalus diaphanus_ (Figs. 5, 6) the main branch of the antenna consists of two large joints, the terminal joint being a strong claw with a serrated process at its base, while the proximal joint bears two appendages on its inner side; one of these is a small, subconical tubercle, the second is more complicated, consisting of a main stem and five outgrowths. The main stem is many-jointed and flexible, its basal joint being longer than the others, and bearing on its outer side a large, triangular, membranous appendage, and four soft cylindrical appendages, the main stem and its appendages being beset with curious tubercles, ending in short spines, whose structure is not understood. Except during the act of copulation this remarkable apparatus is coiled on the inner side of the antennary claw, the jointed stem being so coiled that it is often compared to the coiled proboscis of a butterfly, and the triangular membrane folded like a fan beside it, so that much of the organ is concealed, and the general appearance of the head is that shown in Fig. 5. During copulation, the whole structure is widely extended.

FIG. 7.—_Artemia fertilis._ Front view of the head of a male, showing
the large second antennae, _A.2_; _A.1_, first antennae.
]

The males of _Artemia_ (Fig. 7) have the second antenna two-jointed, the basal joint bearing an inner tubercle, the terminal joint being flattened and bluntly pointed, its outer margin provided with a membranous outgrowth. In _A. fertilis_ the breadth of the second joint varies greatly, the narrower forms presenting a certain remote resemblance to _Branchinecta_. In the males of _Polyartemia_ the second antennae have a remarkable branched form not easily comparable with that found in other Branchipodidae.

The cephalic jaws are fairly uniform throughout the order. The mandibles have an undivided molar surface, and no palp; the first maxilla is very generally a triangular plate, with a setose biting edge; mandibles and maxillae are covered by the labrum. The second maxilla generally lies outside the chamber formed by the labrum, and is a simple oval plate, with or without a special process for the duct of the kidney.

The thoracic limbs, in front of the genital segments, are not as a rule differentiated into anterior maxillipedes and posterior locomotive appendages, as in higher forms; we have seen, however, that all these limbs take part in the prehension of food, and except in the Limnadiidae they all assist in locomotion. One of the middle thoracic legs of _Artemia_ (Fig. 8, A) has a flattened stem, with seven processes on its inner, and two on its outer margin. The gnathobase (_gn_) is large, and fringed with long plumose setae, each of which is jointed; this is followed by four smaller “endites” (or processes on the median side), and then by two larger ones, the terminal endite (the sixth, excluding the gnathobase) being very mobile and attached to the main stem by a definite joint. On the outer side are two processes; a proximal “bract,” a flat plate with crenate edges, partly divided by a constriction into two, and a distal process, cylindrical and vascular, called by Sars and others the “epipodite.” In other Branchipodidae we have essentially the same condition, except that the fifth endite often becomes much larger than in _Artemia_, throwing the terminal endite well over to the outer edge of the limb; such a shift as this, continued farther, might well lead to the condition found in the Limnadiidae, or Apodidae, where the lobe which seems to represent the terminal endite of _Artemia_ is entirely on the outer border of the limb, forming what most writers have called the exopodite (Lankester’s “flabellum”).[18] In the two last-named families the basal exite or bract of the Branchipodidae does not appear to be represented.

FIG. 8.—=A=, Thoracic limb of _Chirocephalus diaphanus_; =B=,
prehensile thoracic limb of male _Estheria_. _gn_, Gnathobase; 1–6,
the more distal endites.
]

The limbs of the Apodidae are remarkable in two ways; those in front of the genital opening (very constantly ten pairs) are not so nearly alike as in most genera of the sub-order, the first two pairs especially having the axis definitely jointed, while the endites are elongated and antenniform; further, while the first eleven segments bear each a single pair of limbs, as is usual among Crustacea, many of the post-genital segments bear several pairs; thus in _Apus cancriformis_ there are thirty-two post-cephalic segments in front of the telson, the first eleven having each one pair of limbs, while the next seventeen have fifty-two pairs between them, the last four segments having none.

In all the Phyllopoda some of the post-cephalic limbs are modified for reproductive purposes; in the Branchipodidae the last two pairs (the 12th and 13th generally, the 20th and 21st in _Polyartemia_) are so modified in both sexes. In the female these appendages fuse at an early period of larval life, and surround the median opening of the generative duct (Fig. 2); in the male the two pairs also fuse, but traces of the limbs are left as eversible processes round the paired openings of the vasa deferentia.

In the other families, one or more limbs of the female are adapted for carrying or supporting the eggs. In the Apodidae the appendages of the eleventh segment have the exopodite in the form of a rounded, watchglass-shaped plate, fitting over a similarly shaped process of the axis of the limb, so that a lens-shaped box is formed, into which the eggs pass from the oviduct. In Limnadiidae the eggs are carried in masses between the body and the carapace, and are kept in position by special elongations of the exopodites of two or three legs, either those near the middle of the thorax (_Estheria_, _Limnadia_), or at its posterior end (_Limnetis_). In female _Limnetis_ the last thoracic segments bear two remarkable lateral plates, which apparently also help to support the eggs. In the male Limnadiidae, the first (_Limnetis_) or the first two thoracic feet (_Limnadia_, _Estheria_) are prehensile (Fig. 8, B).

=Alimentary Canal.=—The mouth of the Phyllopoda is overhung by the large labrum, so that a kind of atrium is formed, outside the mouth itself, in which mastication is performed; numerous unicellular glands, opening on the oral face of the labrum, pour their secretion into the atrial chamber, and may be called salivary, though the nature of their secretion is not known. The mouth has commonly two swollen and setose lips, running longitudinally forwards from the bases of the first maxillae, and often wrapping round the blades of the mandibles. It leads into a vertical oesophagus, which opens into a small globular stomach, lying entirely within the head; the terminal part of the oesophagus is slightly invaginated into the stomach, so that a valvular ring is formed at the junction of the two. The stomach opens widely behind into a straight intestine, which runs backwards to about the level of the telson, where it joins a short rectum, leading to the terminal or ventral anus. The stomach and intestine are lined by a columnar epithelium, and covered by a thin network of circularly arranged muscle-fibres; the rectum has a flatter epithelium, and radial muscles pass from it to the body-wall, so that it can be dilated. The only special digestive glands are two branched glandular tubes, situated entirely within the head, which open into the stomach by large ducts, one on each side. In _Chirocephalus_ the gastric glands are fairly small and simple; in the Apodidae their branches are more complex and form a considerable mass, filling all that portion of the head which is not occupied by the nervous system and the muscles. Backwardly directed gastric glands, like those of the higher Crustacea, are not found in Branchiopods; both forms occur together in the genus _Nebalia_, but with this exception the forwardly-directed glands are peculiar to Branchiopods.

=Heart.=—In _Branchipus_ and its allies, and in _Artemia_, the heart extends from the first thoracic segment to the penultimate segment of the body, and is provided with eighteen pairs of lateral openings, one pair in every segment through which it passes except the last; it is widely open at its hinder end, and is prolonged in front for a short distance as a cephalic aorta, the rest of the blood-spaces being lacunar.

In most, at least, of the other Branchiopods, the heart is closed behind and is shortened; in _Apus_ and _Lepidurus_ it only extends through the first eleven post-cephalic segments, while in the Limnadiidae it is shorter still, the heart of _Limnetis_ passing through four segments only. In all cases there is a pair of lateral openings in every segment traversed by the heart.

The blood of the Branchipodidae and Apodidae contains dissolved haemoglobin, the quantity present being so small as to give but a faint colour to the blood in _Branchipus_, while _Artemia_ has rather more, and the blood of _Apus_ is very red. The only other Crustacea in which the blood contains haemoglobin are the Copepods of the genus _Lernanthropus_,[19] so that the appearance of this substance is as irregular and inexplicable in Crustacea as in Chaetopods and Molluscs.

The =nervous system= of _Branchipus_ may be described as an illustration of the condition prevailing in the group. The brain consists of two closely united ganglia, in each of which three main regions may be distinguished; a ventral anterior lobe, a dorsal anterior lobe, and a posterior lobe. The ventral anterior lobes give off nerves to the median eye, to the dorsal organ, and to a pair of curious sense-organs, comparable with the larval sense-knobs of many higher forms, situated one on each side of the median eye; in late larvae Claus describes the terminal apparatus of each frontal sense-organ as a single large hypodermic cell; W. K. Spencer[20] has lately described several terminal cells, containing peculiar chitinous bodies, in the adult. The homologous sense-organs of _Limnetis_ are apparently olfactory. The dorsal anterior lobes give off the large nerves to the lateral eyes, while the posterior lobes supply the first antennae. The oesophageal connectives have a coating of ganglion-cells, and some of these form the ganglion of the second antenna, the nerve to this appendage leaving the connective just behind the brain. The post-oral nerve-cords are widely separate, each of them dilating into a ganglion opposite every appendage, the two ganglia being connected by two transverse commissures. The ganglia of the three cephalic jaws, so often fused in the higher Crustacea, are here perfectly distinct. Closely connected with each thoracic ganglion is a remarkable unicellular gland, opening to the exterior near the middle ventral line; it is conceivable that these cells may be properly compared with the larval nephridia of a Chaetopod,[21] but no evidence in support of such a comparison has yet been adduced.

Behind the genital segments, where there are no limbs, the nerve-cords run backwards without dilating into segmental ganglia, except in the anterior two abdominal segments where small ganglionic enlargements occur. In Apodidae, on the other hand, those segments which carry more than one pair of appendages have as many pairs of ganglia, united by transverse commissures, as they have limbs.

A stomatogastric nervous system exists in _Apus_, where a nerve arises on each side from the first post-oral commissure, and runs forward to join its fellow of the opposite side on the anterior wall of the oesophagus. From the loop so formed a larger median and a series of smaller lateral nerves pass to the wall of the alimentary canal. A second nerve to the oesophagus is given off from the mandibular ganglion of each side.

=Reproductive Organs.=—In _Chirocephalus_ the ovaries (Fig. 2, _Ov_) are hollow epithelial tubes, lying one on each side of the alimentary canal, and extending from the sixth abdominal segment forwards to the level of the genital opening; at this point the two ovaries are continuous with ducts, which bend sharply downwards and open into the single uterus contained within the projecting egg-pouch and opening to the exterior at the apex of that organ. Short diverticula of the walls of the uterus receive the ducts of groups of unicellular glands, the bodies of which contain a peculiar opaque secretion, said to form the eggshells. In Apodidae the ovaries are similar in structure, but they are much larger and branch in a complex manner, while each ovary opens to the exterior independently of the other in the eleventh post-cephalic segment; nothing like the median uterus of the Branchipodidae being formed. The epithelium of the ovarian tubes proliferates, and groups of cells are formed; one becoming an ovum, the others being nutrient cells like those which will be more fully described in the Cladocera.

In _Chirocephalus_ the testes are tubes similar in shape and position to the ovaries, each communicating in front with a short vas deferens, which dilates into a vesicula seminalis on its way to the eversible penis; an essentially similar arrangement is found in all Branchipodidae, but in Apodidae and Limnadiidae there is no penis.

All the Branchiopoda are dioecious,[22] and many are parthenogenetic. Among Branchipodidae _Artemia_ is the only genus known to be parthenogenetic, but parthenogenesis is common in all Apodidae, while the males of several species of _Limnadia_ are still unknown, although the females are sometimes exceedingly common. In _Artemia_, generations in which the males are about as numerous as the females seem to alternate fairly quickly with others which contain only parthenogenetic females; in _Apus_ males are rarely abundant, and often absent for long periods; during five consecutive years von Siebold failed to discover a male in a locality in Bavaria, though he examined many thousands of individuals; near Breslau he found on one occasion about 11 per cent of males (114 in 1026), but in a subsequent year he found less than 1 per cent; the greatest recorded percentage of males is that observed by Lubbock in 1863, when he found 33 males among 72 individuals taken near Rouen.

The eggs of most genera can resist prolonged periods of desiccation, and indeed it seems necessary for the development of many species that the eggs should be first dried and afterwards placed in water. Many eggs (_e.g._ of _Chirocephalus diaphanus_ and _Branchipus stagnalis_) float when placed in water after desiccation, the development taking place at the surface of the water.

=Habitat.=—All the Phyllopoda, except _Artemia_, are confined to stagnant shallow waters, especially to such ponds as are formed during spring rains, and dry up during the summer. In waters of this kind the species of _Branchipus_, _Apus_, etc., develop rapidly, and produce great numbers of eggs, which are left in the dried mud at the bottom after evaporation of the water, where they remain quiescent until a fresh rainy season. The mud from the beds of such temporary pools often contains large numbers of eggs, which may be carried by wind, on the legs of birds, and by other means, to considerable distances. Many exotic species have been made known to European naturalists by their power of hatching out when mud brought home by travellers is placed in water. The water of stagnant pools quickly dissolves a certain quantity of solid matter from the soil, and often receives dissolved solids through surface drainage from the neighbouring land; such salts may remain as the water evaporates, so that the water which remains after evaporation has proceeded for some time may be very sensibly denser than that in which the Branchiopods were hatched; these creatures must therefore be able to endure a considerable increase in the salinity of the surrounding waters during the course of their lives. My friend Mr. W. W. Fisher points out that the plants present in such a pond would often precipitate the carbonate of lime, so that this might be removed as evaporation went on, but that chlorides would probably remain in solution; from analyses which Mr. Fisher has been kind enough to make for me, it is seen that this happened in a small aquarium in my laboratory, in which _Chirocephalus diaphanus_ lived for four months. In April, mud from the dry bed of a pond, known to contain eggs of _Chirocephalus_, was placed in this aquarium in Oxford, and water was added from the tap. Oxford tap-water contains about 0·3 grm. salts per litre, the chlorine being equivalent to 0·023 grm. NaCl. Water was added from time to time during May and June, but in July evaporation was allowed to proceed unchecked. At the end of July there was about half the original volume of water, the _Chirocephalus_ being still active; the residue contained 0·96 grm. dissolved solids per litre, with chlorine equal to 0·19 grm. NaCl, so that the percentage of chlorides was about eight times the initial percentage, but there were only three and a fifth times the original amount of total solid matter in solution, the carbonate of lime having precipitated as a visible film.

Some species of _Branchipus_ (e.g. _B. spinosus_, M. Edw.) and of _Estheria_ (_E. macgillivrayi_, Baird, _E. gubernator_, Klutzinger) occur in salt pools, but _Artemia_ flourishes in waters beside whose salinity that endured by any other Branchiopod is insignificant. In the South of Europe, _Artemia salina_ may be found in swarms, as it used to be found in Dorsetshire, in the shallow brine-pans from which salt is commercially prepared; Rathke quotes an analysis showing that a pool in the Crimea contained living _Artemia_ when the salts in solution were 271 grms. per litre, and the water was said to have the colour and consistency of beer.

The behaviour of the animals in the water differs a little; in normal feeding all the species swim with the back downwards, as has already been said; the Branchipodidae rarely settle on the ground, or on foreign objects, but the Apodidae occasionally wriggle along the bottom on their ventral surface, and _Estheria_ burrows in mud.

The greater number of species are found in pools in flat, low-lying regions, and many appear to be especially abundant near the sea; _Apus cancriformis_ has, however, been found in Armenia at 10,000 feet above sea level.

Wells and underground waters do not generally contain Phyllopods; but a species of _Branchipus_ and one of _Limnetis_, both blind, have been described from the caves of Carniola.

One of the many puzzles presented by these creatures is the erratic way in which they are scattered through the regions they inhabit; a single small pond, a few yards or less in diameter, may be the only place within many miles in which a given species can be found; in this pond it may, however, appear regularly season after season for some time, and then suddenly vanish.

Geographically, the Phyllopoda are cosmopolitan, representatives of every family and of some genera (e.g. _Streptocephalus_, _Lepidurus_, _Estheria_) being found in every one of the great zoological regions, though a few aberrant genera are of limited range, thus _Polyartemia_ is known only from the northern Palaearctic and Nearctic regions, _Thamnocephalus_ only from the Central United States. The genus _Artemia_ is not at present known in Australia.[23] The only recorded British species are _Chirocephalus diaphanus_, _Artemia salina_, and _Apus cancriformis_,[24] but other continental islands, for example the West Indian group, are better supplied. The distribution of the species is very imperfectly known, but on the whole every main zoological region seems to have its own peculiar species, which do not pass beyond its boundaries. _Branchinecta paludosa_ and _Lepidurus glacialis_ are circumpolar, both occurring in Norway, in Lapland, in Greenland, and in Arctic North America; but with these exceptions the Palaearctic and Nearctic species seem to be distinct. The European species _Apus cancriformis_ occurs in Algiers, but the relations between the species of Northern Africa as a whole and those of Southern Europe on the one hand, or of Central and Southern Africa on the other, have yet to be worked out.

The soft-bodied Branchipodidae are not known in the fossil condition;[25] an _Apus_, closely related to the modern _A. cancriformis_, has been found in the Trias, but the most numerous remains have been left, as might be expected, by the hard-shelled Limnadiidae; carapaces, closely resembling those of the modern _Estheria_, are known in beds of all ages from the Devonian period to recent times; these carapaces are in several cases associated with fossils of an apparently marine type. None of the fossil species differ in any important characters from those now living, so that the Phyllopoda have existed in practically their present form for an enormously long period; this fact, and the evidence that species of existing genera were at one time marine, explain the wide distribution of animals at present restricted to a remarkably limited range of environmental conditions.

=Summary of the Characters of the Genera.=

SUB-ORDER PHYLLOPODA.—Branchiopoda with an elongated body, provided
with at least ten pairs of post-cephalic limbs, the heart extending
through four or more thoracic segments, and having at least four
pairs of ostia.

=Fam. 1. Branchipodidae.=[26]—Carapace rudimentary, eyes stalked; the
second antennae flat and unjointed in the female, jointed and
prehensile in the male; female generative opening single; telson not
laterally compressed, bearing two flattened lobes, or none. The
heart extending through the thorax and the greater part of the
abdomen.

A. Eleven pairs of praegenital ambulatory limbs.

_a._ Abdomen of six well-formed segments and a telson; anal
lobes well formed, their margins setose.

_Branchinecta_, Verrill—Second antennae of ♂ without
lateral appendages; ovisac of ♀ elongated. _B.
paludosa_, O. F. Müll.—Circumpolar.

_Branchiopodopsis_, G. O. Sars[27]—Second antennae of ♂ as
in _Branchinecta_; ovisac of ♀ short. _B. hodgsoni_, G.
O. Sars—Cape of Good Hope.

_Branchipus_, Schaeffer—Second antennae of ♂ with simple
internal filamentous appendage. _B. stagnalis_, Linn.—
Central Europe.

_Streptocephalus_, Baird—Second antennae of ♂ 3–jointed,
the last joint bifid; an external filamentous appendage.
_S. torvicornis_, Wagn., Poland.

_Chirocephalus_, Prévost—Second antennae of ♂ 3–jointed,
with a jointed internal appendage, which bears secondary
processes, four cylindrical and one lamellar. _C.
diaphanus_, Prévost (Fig. 2, p. 20).—Britain, Central
Europe.

_b._ Abdominal segments five or fewer, and a telson. Anal
lobes small or 0, sparsely or not at all setose.

_Artemia_, Leach—Second antennae of ♂ without filamentous
appendage, 2–jointed, the second joint lamellar. _A.
salina_, Linn.—Brine pools of the Palaearctic region.

_c._ Hinder abdominal segments united with telson to form a
fin; anal lobes absent.

_Thamnocephalus_, Packard—Head with a branched median
process of unknown nature. Only species _T. platyurus_,
Packard—Kansas, U.S.A.

B. Nineteen pairs of praegenital ambulatory limbs.

_Polyartemia_, Fischer—Second antennae of ♂ forcipate;
ovisac of ♀ very short. Only species _P. forcipata_,
Fisch.

=Fam. 2. Apodidae.=[28]—Carapace well developed as a depressed
shield, covering at least half the body. Eyes sessile, covered; no
male clasping organs; anal lobes long, jointed cirri.

_Apus_, Scopoli—Telson not produced backwards over the
anus; endites of first thoracic limb very long. _A.
cancriformis_, Schaeffer—Britain, Europe, Algiers,
Tunis. _A. australiensis_, Central Australia.

_Lepidurus_, Leach—Telson produced backwards to form a
plate above the anus; endites of first thoracic limb
short. _L. productus_, Bosc.—Central Europe. _L.
viridis_, Southern Australia, New Zealand, _L.
patagonicus_, Bergh, Argentines.

=Fam. 3. Limnadiidae.=—Body compressed; carapace in the form of a
bivalve shell, the two halves capable of adduction by means of a
strong transverse muscle; second antennae biramous, alike in both
sexes; in the male, the first or the first and second thoracic limbs
prehensile; telson laterally compressed.

A. Only the first thoracic limbs prehensile in the male; the
carapace spheroidal, without lines of growth; head not included
within the carapace-chamber.

_Limnetis_, Lovén—Compound eyes fused; anal spines absent;
ambulatory limbs 10–12. _L. brachyura_, O. F. Müll (Fig.
3, p. 21).—Norway, Central Europe.

B. The first and second thoracic limbs prehensile in the male;
carapace distinctly bivalve, enclosing the head, with concentric
lines of growth round a more or less prominent umbo.

_Eulimnadia_, Packard—Carapace narrowly ovate, with few
(4–5) lines of growth. _E. mauritani_, Guérin—Mauritius.
_E. texana_, Packard—Texas, Kansas.

_Limnadia_, Brongniart—Carapace broadly ovate, with
numerous lines of growth, without distinct umbones; _L.
lenticularis_, Linn.—Northern and Central Europe.

_Estheria_, Rüppell—Carapace with well-marked umbones and
numerous lines of growth, oval; _E. tetraceros_,
Kryneki—Central Europe.

_Leptestheria_,[29] G. O. Sars—Carapace compressed,
oblong. Rostrum with a movable spine; thoracic limbs
with accessory lappet on the exopodite. _L. siliqua_, G.
O. Sars—Cape Town.

_Cyclestheria_,[30] G. O. Sars. _C. hislopi_, Baird—
Queensland, India, East Africa, Brazil.

=Sub-Order 2. Cladocera.=

The Cladocera are short-bodied Branchiopods, with not more than six pairs of thoracic limbs. The second antennae are important organs of locomotion, and are nearly always biramous; the first antennae are small, at least in the female; the second maxillae are absent in the adult. The carapace may extend backwards so as to enclose the whole post-cephalic portion of the body, or may be reduced to a small dorsal brood-pouch, leaving the body uncovered.

The Cladocera or “Water-fleas” are never of great size; _Leptodora hyalina_, the largest, is only about 15 mm. long, while many Lynceidae are not more than 0·1 or 0·2 mm. in length.

The =head= is bent downwards in all the Cladocera, so that parts which are morphologically anterior, such as the median eye and the first antennae, lie ventral to or even behind the compound eyes and the second antennae (_cf._ Fig. 10).

The compound lateral eyes fuse at an early period of embryonic life, so that they form a single median mass in the adult, over which a fold of ectoderm grows, to make a chamber over the eye, like that found in the Limnadiidae, except that it is completely closed. The fused eyes are generally large and conspicuous; in some deep-water forms the retinular elements of the dorsal portion are larger than those of the ventral (e.g. _Bythotrephes_, Fig. 13). In one or two species which live at very great depths, or in caves, the eyes are altogether absent.

The appendages of the head are fairly uniform, the most variable being the first antennae. In the females of many genera the first antennae are short and immovable, consisting of a single joint, with a terminal bunch of sensory hairs, and often a long lateral hair, as in _Simocephalus_ (Figs. 9, 10), _Daphnia_, etc. In the female _Moina_ (Fig. 16) they are movable, as they are in _Ceriodaphnia_ and some others; in _Bosmina_ (Fig. 22) and many Lyncodaphniidae they are elongated and imperfectly divided into joints by rings of spines, while in _Macrothrix_ they are flattened plates. In the males the first antennae are elongated and mobile (_cf._ Figs. 11, 19).

FIG. 9.—_Simocephalus vetulus_, female. Ventral view, without the
carapace; _A_{1}_, _A_{2}_, first and second antennae; _For_, head;
_Md_, mandible; _Te_, telson; I-IV, first to fourth thoracic
appendages.
]

The second antennae, the chief organs of locomotion, are biramous in all genera except _Holopedium_; the number of joints in each ramus, and the number of the long plumose hairs with which they are provided, are remarkably constant in whole series of genera, and are therefore useful for purposes of classification. The creatures row themselves by quick strokes of these appendages, the movement being slow and irregular in the rounder forms, such as _Simocephalus_ or _Daphnia_, rapid and well directed in such elongated lacustrine forms as _Bythotrephes_ or _Leptodora_.

The mandibles have no palp; the first maxillae are very small, and the second maxillae are absent (Fig. 9).

The =carapace= varies very much. In most genera (the CALYPTOMERA of Sars) it is a large, backwardly-projecting fold of skin, bent downwards at the sides so as to form a bivalve shell, enclosing the whole post-cephalic portion of the body, as in _Simocephalus_ (Fig. 10). The eggs are laid into the space between the carapace and the dorsal part of the thorax, both the carapace and the thorax itself being often modified for their protection and nutrition. In a few forms, the GYMNOMERA of Sars, the carapace serves only as a brood-pouch, which is distended when eggs are laid, but collapses to an inconspicuous appendage at the back of the head when it is empty (e.g. _Leptodora_, Fig. 24, _Bythotrephes_, Fig. 13). In the Calyptomera the surface of the carapace is frequently provided with a series of ridges, which may be parallel, rarely branching, as in _Simocephalus_; or in two sets which cross nearly at right angles, as in _Daphnia_; or so arranged as to form a hexagonal pattern, as in _Ceriodaphnia_. In a few forms the whole surface is irregularly covered with spines or scales. The hinder edge of the carapace is often produced into a median dorsal spine (_Daphnia_, Fig. 19), or more rarely there are two spines, one at each ventro-lateral corner (_Scapholeberis_, Fig. 20).

FIG. 10.—_Simocephalus vetulus_, × 30. Side view of female, showing
the arrangement of the principal organs. _A.2_, Second antenna;
_C.S_, cervical suture; _E_, fused compound eyes; _H_, heart; _L_,
forwardly-directed gastric caeca; _N_, dorsal organ.
]

The cuticle of the carapace is often separated from that of the head by a cervical suture, as in _Simocephalus_ (Fig. 10, _C.S._) and near the line of demarcation many forms exhibit patches of glandular ectoderm which seem to be homologous with the dorsal adhesive organs of the Limnadiidae. The commonest condition is that of a median dorsal pit (Fig. 10, _N._) by means of which the animal can fix itself to foreign objects. Certain forms may remain for long periods of time attached by the dorsal organ to plants, or to the sides of an aquarium, the only movement being a slow vibration of the feet, by which a current of water, sufficiently rapid for respiratory purposes, is established round it.[31] In _Sida crystallina_ (Fig. 11) the dorsal organ is represented by three structures; in front there is a median raised patch (_N.m_) of columnar ectoderm, containing concretions like those described in the Branchipodidae, and behind this is a pair of cup-shaped organs (_N.e_), with raised margins.

FIG. 11.—_Sida crystallina_, male, × 27. Oxford. _A.1_, Elongated
first antenna; _N.e_, paired element of dorsal organ; _N.m_, median
element of dorsal organ; _Te_, testes; ♂, opening of vas deferens.
]

The fold of skin which forms the carapace contains the coils of the single pair of kidneys, and it forms an important organ of respiration, partly from the great size of the blood-vessels it contains, and partly from the presence of red, blue, or brown respiratory pigments in the tissue of the skin itself.

In most Cladocera the cuticle of the carapace is cast at every ecdysis, with that of other parts of the body; but in _Iliocryptus_ and a few others it remains after each moult, giving the carapace an appearance of “lines of growth,” like that seen in many Limnadiidae.

The segmentation of the =body= behind the head is obscure, but we can generally recognise (1) a thorax, of as many segments as there are pairs of limbs; (2) an abdomen of three segments; and (3) a telson.

The thoracic limbs of the Calyptomera are flattened, and resemble those of the Phyllopoda; as a type we may examine the third thoracic limb of _Simocephalus_ (Fig. 12, C), in which the axis bears a large setose gnathobase (_Gn_) on its inner edge, followed by two small endites; the terminal process, or exopodite (_Ex_) is a large flattened plate, with six long plumose hairs on its edge. The outer margin of the axis bears a bract (_Br_) and an epipodite.

In _Simocephalus_, as in the other Daphniidae, there are five pairs of thoracic limbs, of which the third and fourth are alike; in the female each limb of the first pair consists of a jointed axis, with strong biting hairs on the inner border, and a rudimentary epipodite (Fig. 12, A), the second limb being more like the third, but with a more prominent gnathobase and a narrower exopodite (B), while the limbs of the fifth pair have the gnathobase and the exopodite filamentous (D).

In the Sididae there are six pairs of thoracic limbs, which are nearly alike in the female; in the Bosminidae there are six pairs, the first two modified for prehension, the last much reduced.

FIG. 12.—Thoracic limbs of female _Simocephalus vetulus_. =A=, The
first; =B=, the second; =C=, the third; =D=, the fifth. _Br_, Bract;
_Ep_, epipodite; _Ex_, exopodite; _Gn_, gnathobase.
]

In the male, the first thoracic limb is usually provided with a long sensory process and a prehensible hook (Figs. 11, 19).

In the Gymnomera the limbs are cylindrical, jointed rods, with a gnathobase on the inner side in the Polyphemidae, but not in _Leptodora_. The number varies from four to six pairs.

The abdomen bears no appendages. The telson is compressed in the Calyptomera, and is produced into two flattened plates, one on each side of the anal opening. The backwardly directed margins of these plates are commonly serrated, and the lower corner of each is produced into a curved spine, which carries secondary teeth. The number and arrangement of these teeth, though often extremely variable in the same species, are used extensively as specific characters. Above the anus the telson commonly bears two long plumose hairs, which are directed backwards.

FIG. 13.—_Bythotrephes cederströmii_, female, × 20, North Wales, from
a specimen found by A. D. Darbishire. _Car_, carapace.
]

In the Gymnomera the telson is not bilaterally compressed, and it may be produced into a long spine, dorsal to the anus (e.g. _Bythotrephes_, Fig. 13).

The =alimentary canal= is extremely simple. The labrum is large, and forms a chamber above the mouth, into which food is driven by the limbs, as in the Phyllopoda, food being taken while the animal swims or lies on its back. The oesophagus runs vertically to join a small stomach, which bends sharply backwards and passes gradually into an intestine. In the last segment of the abdomen the intestine joins a short, thin-walled rectum, provided with radial muscles, by means of which it can be dilated. The dilatation of the rectum leads to an inhalation of water through the anus, which may possibly serve as a means of respiration. In the Daphniidae and Bosminidae there are two forwardly-directed digestive glands which open into the stomach, and in _Eurycercus_ there is a large caecum at the junction of the rectum with the intestine. The intestine is usually straight, but in Lynceidae and in some Lyncodaphniidae it is coiled (e.g. _Peracantha_, Fig. 14).

In _Leptodora_ the alimentary canal is altogether remarkable; the oesophagus is a long and very narrow tube, which runs back through the whole length of the thorax and joins the mid-gut in the third abdominal segment. The mid-gut is not differentiated into stomach and intestine; it has no diverticula of any kind, and runs straight backwards to join the short rectum a little in front of the anus.

FIG. 14.—_Peracantha truncata_, female, × 100. Oxford.
]

The =heart= is always short, and never has more than a single pair of lateral openings; it is longest in the Sididae, which show some approximation to the Phyllopods in this, as in the slight degree of difference between their anterior and posterior thoracic limbs. The pericardium lies in the one or two anterior thoracic segments, dorsal to the gut. From the heart the blood runs forwards to the dorsal part of the head, and passes backwards by three main channels, one entering each side of the carapace, while the third runs down the body, beneath the alimentary canal to dilate into a large sinus round the rectum. This ventral blood-channel gives a branch to each limb, which forms a considerable dilatation in the epipodite, the blood from the limb returning to the pericardium by a lateral sinus. From the rectum a large sinus runs forwards to the pericardium along the dorsal wall of the body. The blood which enters each half of the carapace is collected in a median vessel and returned through this to the pericardium.

Those spaces between the viscera which are not filled with blood are occupied by a peculiar connective tissue, consisting of rounded or polyhedral cells, charged with drops of a fatty material which is often brightly coloured.

The =reproductive organs= are interesting because of the peculiar phenomena connected with the nutrition of the two kinds of eggs. The ovaries or testes are epithelial sacs, one on each side of the body, each continuous with a duct which opens to the exterior behind the last thoracic limb. In the female, the opening is dorsal (Fig. 10), in the male it is ventral (Fig. 11). The external opening is usually simple; but in the male there is sometimes a penis-like process, on which the vas deferens opens (_Daphnella_).

The eggs are of two kinds, the so-called “summer-eggs,” with relatively little yolk, which develop rapidly without fertilisation, and the so-called “winter-eggs,” containing much yolk, which require to be fertilised and then develop slowly.

At one end of the ovary, generally that nearest to the oviduct, there is a mass of protoplasm, containing nuclei which actively divide; this is the germarium (Fig. 15, A, B, C). As a result of proliferation in the germarium, nucleated masses are thrown off into the cavity of the ovary; each such mass contains four nuclei, and its protoplasm soon becomes divided into four portions, one round each nucleus, so that four cells are produced. In the simpler ovaries, such as that of _Leptodora_ (Fig. 15, A), these sets of four cells are arranged in a linear series within the tube of ovarian epithelium; in other cases, as in _Daphnia_, the arrangement is more irregular. In the normal development of parthenogenetic eggs, one cell out of each set of four becomes an ovum, the other three feeding it with yolk and then dying. Weismann[32] has shown that the ovum is always formed from the third cell of each set, counting from the germarial end, so that in the ovary of _Leptodora_ drawn in Fig. 15, A, the ova will be formed from the cells marked E_{1}, E_{2}, E_{3}. At certain times, one or two sets of germinal cells fail to produce ova; the epithelial wall of the ovary thickens round these cells, so that they become incompletely separated from the rest in a so-called “nutrient chamber” (Fig. 15, B, _N.C_). Germ-cells enclosed in a nutrient chamber degenerate and are ultimately devoured by the ovarian epithelium. The significance of these nutrient chambers is unknown.

FIG. 15.—=A=, Ovary of a parthenogenetic _Leptodora hyalina_; =B=,
base of another ovary of the same species, showing a so-called
“nutrient chamber”; =C=, ovary of a female _Daphnia_, showing the
formation of a winter-egg. _E_, _E_{1}–E_{3}_, Parthenogenetic egg;
_Ep_, ovarian epithelium; _G_, germarium; _N.C_, nutrient chamber;
_O.D_, oviduct; _W_, winter-egg; 1, 2, 4, the other three cells of
the same group; II, III, two other groups of germ-cells.
]

The production of a winter-egg is a more complicated process. The epithelium of the ovarian tube swells up, so that the lumen is nearly obliterated, and several sets of four germ-cells pass from the germarium to lie among the swollen epithelial cells. All these groups of germ-cells, except one, disintegrate and are devoured by the ovarian epithelium, one cell of the remaining group enlarging to form a winter-egg, fed during its growth not only by the three cells of its own set but also by the epithelial cells of the ovarian tube, which have devoured the germ-cells of other sets. An ovary never contains more than a single winter-egg at the same time, the number of germ-cells which are devoured during its formation varying in the different species; the _Daphnia_ drawn in Fig. 15, C, has produced three groups of germ-cells, of which two (II, III), will die, while the cell W from the remaining group will develop into an ovum; in _Moina_, Weismann finds that as many as a dozen cell-groups may be thrown into the ovary before the production of a winter-egg, so that only one out of forty-eight germ-cells survives as an ovum.

FIG. 16.—Sketch of a parthenogenetic _Moina rectirostris_, × 45, the
brood-pouch being emptied and the side of the carapace removed,
showing the dome of thickened epithelium on the thorax, by which
nutrient material is thrown into the brood-pouch, and the ridge
which fits against the carapace in the natural condition so as to
close the brood-pouch.
]

FIG. 17.—_Moina rectirostris_, ♀, × 40, showing the ephippial
thickening of the carapace which precedes the laying of a
winter-egg.
]

The summer-eggs are always carried until they are hatched by the parthenogenetic female which produces them. The brood-pouch is the space between the dorsal wall of the thorax and the carapace. This space is always more or less perfectly closed at the sides by the pressure of the carapace against the body, and behind by vascular processes from the abdominal segments (Figs. 10, 16, etc.). The presence of a large blood-sinus beneath the dorsal wall of the thorax and in the middle line of the carapace suggests the possibility that some special nutrient substances may pass from the body of the parent into the brood-chamber, and in some species the thoracic ectoderm is specially modified as a placenta. In _Moina_ (Fig. 16) the dorsal wall of the thorax is produced into a dome, covered by a columnar ectoderm, which contains a dilatation of the dorsal blood-sinus; and in this form it has been shown that the fluid in the brood-pouch contains dissolved proteids. Associated with the apparatus for supplying the brood-pouch with nutriment is a special apparatus for closing it, in the form of a raised ridge, which projects from the back and sides of the thorax and fits into a groove of the carapace.

A somewhat similar nutrient apparatus exists in the Polyphemidae, where the edges of the small carapace are fused with the thorax, so that the brood-pouch is completely closed, and the young can only escape when the parent casts her cuticle. In some genera of this family (e.g. _Evadne_) the young remain in the parental brood-pouch until they are themselves mature, so that when they are set free they may already bear parthenogenetic embryos in their own brood-pouches.

FIG. 18.—Newly-cast ephippium of _Daphnia_, containing two
winter-eggs.
]

The winter-eggs are fertilised in the same part of the carapace of the female in which the parthenogenetic eggs develop, but after fertilisation they are thrown off from the body of the mother, either with or without a protective envelope formed from the cuticle of the carapace. The eggs of _Sida_ are surrounded by a thin layer of a sticky substance, and when cast out of the maternal carapace they adhere to foreign objects, such as water-weeds; those of _Polyphemus_ have a thick, gelatinous coat; in _Leptodora_ and _Bythotrephes_ the egg secretes a two-layered chitinous shell. In these forms the cuticle of the parent is not used as a protection for the winter-eggs, although it is generally, if not invariably, thrown off when the eggs are laid. In the Lynceidae the cuticle is moulted in such a way that the winter-eggs remain within it, at least for a time; the cuticle is occasionally modified before it is thrown off; thus in _Camptocercus macrurus_ the cuticle of the carapace, in the region of the brood-pouch, becomes thickened and darkly coloured, forming a fairly strong case round the eggs. The modification of the cuticle round the brood-pouch is much more pronounced in the Daphniidae, where it leads to the formation of a saddle-shaped cuticular box, the “ephippium,” in which the winter-eggs are enclosed. The ripening of a winter-egg in the ovary of a _Daphnia_ is accompanied by a great thickening of the cuticle of the carapace (_cf._ Fig. 18), so that a strong case is formed in the position of the brood-pouch. The winter-eggs are laid between the two valves of this case, and shortly afterwards the parent moults. The eggs are retained within the ephippium, from which the rest of the cuticle breaks away (Fig. 18). After separation, the ephippium, which contains a single egg (_Moina rectirostris_) or usually two (_Daphnia_, etc.), either sinks to the bottom, as in _Moina_, or floats.

The winter-eggs usually go through the early stages of segmentation within a short time after they are laid, but after this a longer or shorter period of quiescence occurs, during which the eggs may be dried or frozen without injury. The sides and floor of a dried-up pond are often crowded with ephippia, containing winter-eggs which develop quickly when replaced in water; and the resting-stage of winter-eggs produced in aquaria can often be materially shortened by drying the ephippia which contain them, though such desiccation does not appear to be necessary for development. Under normal conditions large numbers of winter-eggs remain quiescent through the winter and hatch in the following spring.

The individual developed from a sexually fertilised winter-egg is invariably a parthenogenetic female: the characters of the succeeding generations differ in different cases.

In a few forms, of which _Moina_ is the best known, the parthenogenetic female, produced from a winter-egg, may give rise to males, to sexual females, and to parthenogenetic females, so that the cycle of forms which intervene between one winter-egg and the next is short. A sexual female produces one or two winter-eggs, and if these are fertilised they are enclosed in an ephippium and cast off; if, however, the eggs when ripe are not fertilised, they atrophy, and the female produces parthenogenetic eggs, being thenceforward incapable of forming sexual “winter” eggs. An accidental absence of males may thus lead to the occurrence of parthenogenesis in the whole of the second generation. The regular production of sexual individuals in the second generation from the winter-egg appears to depend on a variety of circumstances not yet understood. Mr. G. H. Grosvenor tells me that _Moina_ from the neighbourhood of Oxford may give rise to several successive generations of parthenogenetic individuals, when grown in small aquaria.

In the greater number of Daphniidae, the parthenogenetic female, produced from a winter-egg, gives rise only to parthenogenetic forms, and it is not until after half a dozen parthenogenetic generations have been produced that a few sexual forms appear, mixed with the others. Such sexual forms are fairly common in April or May in this country; they produce “winter” eggs and then die, the generations which succeed them through the summer being entirely parthenogenetic. In late autumn sexual individuals are again produced, giving rise to a plentiful crop of winter-eggs, but many parthenogenetic females are still found, and some of these appear to live and to reproduce through the winter.

In _Sida_, in the Polyphemidae and Leptodoridae, and in most of the Lynceidae, sexual individuals are produced only once in every year, while in a few forms which inhabit great lakes the sexual condition occurs so rarely that it is still unknown.

Weismann[33] has pointed out that the sexual forms, with their property of producing eggs which can endure desiccation, recur most frequently in species such as _Moina_, which inhabit small pools liable to be dried up at frequent intervals, while the species which produce sexual forms only once a year are all inhabitants either of great lakes which are never dry, or of the sea. Many suggestions have been made as to the environmental stimulus which induces the production of sexual individuals, but nothing is definitely known upon the subject.

We have said that even in those generations which contain sexual males and females there are always some parthenogenetic individuals; there is therefore nothing in the behaviour of Daphniidae, either under natural conditions or when observed in aquaria, to suggest that there is any natural or necessary limit to the number of generations which may be parthenogenetically produced.

The parthenogenetic Daphniidae are extremely sensitive to changes in their surroundings; small variations in the character and amount of substances dissolved in the water are often followed by changes in the length of the posterior spine, in the shape and size of crests on the head, and in other characters affecting the appearance of the creatures, so that the determination of species is often a matter of great difficulty. It is remarkable that the green light which has passed through the leaves of water-plants appears to have a prejudicial effect upon some species. Warren has shown that _Daphnia magna_ reproduces more slowly when exposed to green light, and that individuals grown in this way are more readily susceptible to injury from the presence of small quantities of salt (sodium chloride) in the water than individuals which have been exposed to white light.

The majority of the Cladocera belong to the floating fauna of the fresh waters and seas; a few are littoral in their habits, clinging to water-weeds near the shore, a very few live near the bottom at considerable depths, but the majority belong to that floating fauna to which Haeckel gave the name of “plankton.” The Crustacea are an important element in the plankton, whether in fresh waters or in the sea, the two great groups which contribute most largely to it being the Cladocera and the Copepoda. For this reason it will be more convenient to discuss the habits and distribution of individual Cladocera and Copepoda together in a chapter specially devoted to the characters of pelagic faunas (_cf._ Chap. VII.). We will only add to the present chapter a table of the families with a diagnosis of the British genera.

=Summary of Characters of the British Genera.=[34]

Tribe I. CALYPTOMERA, Sars.—The post-cephalic portion of the body enveloped in a free fold or carapace.

A. Six pairs of thoracic feet, the first pair not prehensile
(CTENOPODA).

=Fam. 1. Sididae=: second antennae biramous in both sexes. _Sida_,
Straus (Fig. 11): second antenna with three joints in the dorsal
ramus, two in the ventral; the rostrum large, the teeth on the
telson many. _Latona_, Straus: second antenna with two joints in the
dorsal ramus, three in the ventral, the proximal joint of the dorsal
ramus provided with a setose appendage. _Daphnella_, Baird: second
antenna with the joints as in _Latona_, but with no setose
appendage.

=Fam. 2. Holopediidae=: second antennae not biramous in the female; a
rudimentary second ramus in the male. _Holopedium_, Zaddach.

B. Four to five or six pairs of thoracic feet, the anterior pair
prehensile (ANOMOPODA).

=A.= Ventral ramus of second antenna with three joints, the dorsal
ramus with four.

=Fam. 3. Daphniidae=: five pairs of thoracic feet, with a gap between
the fourth and fifth pairs. The stomach with two forwardly-directed
diverticula.

FIG. 19.—_Daphnia obtusa_, male, × about 50. Oxford. _A._1, First
antenna; _Th._1, first thoracic appendage.
]

i. First antennae of female short.

α A median dorsal spine on posterior margin of carapace.
_Daphnia_, O. F. Müller (Fig. 19): first antennae of female not
mobile. The head separated from the thorax only by a slight
constriction or not at all. Cuticle with a quadrate rhomboid
pattern. _Ceriodaphnia_, Dana: first antennae of female mobile.
The head separated by a deep depression from the thorax. Cuticle
with a polygonal pattern.

β A pair of ventral spines on posterior margin of carapace.
_Scapholeberis_, Schoedler (Fig. 20).

FIG. 20.—_Scapholeberis mucronata_, female, × 25. Oxford.
]

γ No spine on posterior margin of carapace. _Simocephalus_,
Schoedler (Fig. 10, p. 39): the cuticle with a pattern of
parallel branching ridges.

FIG. 21.—_Moina rectirostris_, female, × 24. Oxford.
]

ii. First antennae of female long, mobile. _Moina_, Baird (Figs. 16,
17, 21): median eye absent. Posterior margin of carapace without a
spine.

FIG. 22.—_Bosmina_ sp., female, × about 80. Lake Constance.
]

FIG. 23.—_Acroperus leucocephalus_, × about 35. Oxford.
]

=Fam. 4. Bosminidae=: feet equidistant, five or six pairs; the first
antennae of the female immobile, with sense-hairs arranged in rings,
not forming an apical tuft. The intestine uncoiled; no caeca.
_Bosmina_, Baird (Fig. 22).

=Fam. 5. Lyncodaphniidae=: four, five, or six pairs of equidistant
thoracic limbs; the first two pairs prehensile. First antennae of
female mobile, with apical sense-hairs. Intestine coiled or
straight.

i. Four pairs of thoracic limbs. _Lathonura_, Lilljeborg.

ii. Five pairs of thoracic limbs.

_a._ The four-jointed ramus of the second antenna with four
swimming hairs. _Macrothrix_, Baird: the first antennae of the
female flattened, curved. The intestine simple, straight.
_Streblocerus_, Sars: first antennae of the female very little
flattened, curved backwards and outwards. The intestine
coiled, the stomach with two forwardly-directed caeca.

_b._ The four-jointed ramus of the second antenna with only
three swimming hairs. _Drepanothrix_, Sars.

iii. Six pairs of thoracic limbs; the labrum provided with an
appendage. _Acantholeberis_, Lilljeborg: appendage of labrum
long, pointed, and setose. Intestine without caecum.
_Ilyocryptus_, Sars: appendage of the labrum short, truncated.
Intestine with a caecum.

=B.= Both rami of second antenna three-jointed.

=Fam. 6. Lynceidae=[35]: five or six equidistant pairs of
thoracic feet. Intestine coiled.

i. Six pairs of thoracic limbs. Head and thorax separated by a
deep depression. Intestine with one caecum, stomach with
two. Female carries many summer-eggs. _Eurycercus_, Baird.

ii. Five pairs of thoracic limbs. Head and thorax separated by
a slight groove or not at all. Anterior digestive caeca
absent. Female carries only one or two summer-eggs.

A. Body elongate, oval.

_a._ Head carinate, the eye far from the anterior cephalic
margin. _Camptocercus_, Baird: body laterally compressed.
Second antennae with seven swimming hairs. Telson more than
half as long as the shell. _Acroperus_, Baird (Fig. 23):
body compressed. Second antennae with eight swimming hairs,
of which one is very small. Telson less than half as long as
the shell.

_b._ Head not carinate, the eye near the anterior cephalic
margin. _Alonopsis_, Sars: terminal claws of telson with
_three_ accessory teeth. _Alona_, Baird: terminal claws of
telson with one accessory tooth (includes sub-genera
_Leydigia_, _Alona_, _Harporhynchus_, _Graptoleberis_).
_Peracantha_, Baird (Fig. 14): terminal claws of telson with
two accessory teeth (includes sub-genera _Alonella_,
_Pleuroxus_, _Peracantha_).

B. Body small, spheroidal; the head depressed. _Chydorus_,
Leach: compound eye present. _Monopsilus_, Sars: compound
eye absent.

Tribe II. GYMNOMERA, Sars.—The carapace forms a closed brood-pouch, which does not cover the body; all the thoracic limbs prehensile.

=Fam. 7. Polyphemidae=: four pairs of thoracic limbs, provided with a
gnathobase.

Fresh-water genera.—_Polyphemus_, Müller, with no rudimentary exites
on first three thoracic limbs. _Bythotrephes_, Leydig (Fig. 13),
with no trace of processes on the outer sides of the limbs.

Marine genera.—_Evadne_, Lovén, the head not separated by a
constriction from the thorax. _Podon_, Lovén, with deep cervical
constriction.

FIG. 24.—_Leptodora hyalina_, × 6. Lake Bassenthwaite. _A._1, First
antenna; _Car_, carapace; I, VI, first and sixth thoracic
appendages.
]

=Fam. 8. Leptodoridae=: six pairs of thoracic limbs, with no
gnathobase. Only genus, _Leptodora_, Lilljeborg (Fig. 24), from
fresh water.

_Note._—For extra-European Cladocera consult Daday, “Microskopische
Süsswassertiere aus Patagonien und Chili,” _Termés Füzetek_, xxv.,
1902, p. 201; for Paraguay, _Bibliotheca Zoologica_, Heft 44; for
Ceylon, _Termés Füzetek_, xxi., 1898; and for Australia, Sars,
_Christiania Vidensk. Forhand._ 1885, No. 8, and 1888, No. 7; and
_Arch. f. Math. og Naturvid._ xviii., 1896, No. 3, and xix., 1897, No.
1.—G. W. S.