Chapter XIII: Introduction: Nematoda—anatomy—embryology—classification—ascaridae (3)

FAMILY I. ECHINORHYNCHIDAE.—The body is elongated and smooth. The proboscis-sheath has a double wall, and the proboscis is invaginated into it. The central nerve-ganglion lies in the middle line, as a rule on the posterior blind end of the proboscis-sheath. The papillae which bear the hooks are only covered with a chitinous cap at their apex, and the hooks have a process below. This family is by far the largest; a few species only can be mentioned. _Echinorhynchus proteus_ lives in its mature form in fishes; the young forms, up to a centimetre in length, are found living freely in the intestine of numerous fresh-water fishes. Those found in _Gobio fluviatilis_, the gudgeon; _Leuciscus virgo_; _Lota vulgaris_, the burbot or eel-pout; young trout; _Thymallus vulgaris_, the grayling, seldom surpass this size, but those found in _Acerina cernua_, the pope fish; in _Abramis bipunctatus_; in _Esox lucius_,the pike, and in older trout, attain or surpass double the length. As the parasites grow older they bury their proboscis and neck in the wall of the intestine, the inner surface of which is studded with the orange-coloured bodies of the parasites. The proboscis is so deeply sunk in the wall of the alimentary canal as to form a papilla on its outer surface (Fig. 92). The larvae of _E. proteus_ are found in the body-cavity of _Gammarus pulex_, one of the Amphipod Crustacea, and also in the same position in numerous fresh-water fishes; they must have passed into this first host by the {183}mouth and alimentary canal. If the liver of an infested minnow, _Leuciscus phoxinus_, be examined, it will be found to contain on its surface numerous spherical or egg-shaped capsules of an orange colour, 2 to 2.5 mm. in length; these contain the larval forms of the parasite. They develop into the adult form when the first host is eaten by a carnivorous fish, but a complication may take place when the larval form is found in _Gammarus_, as the latter, the first host, may be eaten by a fish (intermediate host) in which the larva does not become mature, and only develops sexual organs when eaten by a carnivorous fish (second host). The larval form is also found in _Nemachilus barbatulus_, _Gobio fluviatilis_, and the sticklebacks _Gasterosteus aculeatus_ and _G. pungitius_.

_E. clavula_ Duj. is found in _Salmo fario_, _Abramis brama_, _Cyprinus carpio_, _Gobius niger_, _Lepadogaster gouanii_, etc.; _E. linstowi_ Ham. in _Leuciscus idus_, _Abramis ballerus_, _Abramis bipunctatus_, and _Acipenser huso_; _E. lutzii_ Ham. was found by Dr. Lutz in Brazil in the intestine of _Bufo agua_; _E. angustatus_ Rud. occurs in such numbers in the perch, _Perca fluviatilis_, as to almost occlude the lumen of the intestine, and one out of every three or four fish in certain districts is infested by it. It is also found in the pike, _Esox lucius_, and the barbel, _Barbus vulgaris_. The first or larval host of this species is the Isopod _Asellus aquaticus_. _E. moniliformis_ Brews. is stated to attain maturity in the human intestine. Except for the fact that _G. gigas_ has once been observed in the same place, this is the only human parasite amongst the Acanthocephala. Its normal second hosts are _Mus decumanus_ and _Myoxus quercinus_, and its first or larval host, the larvae of the beetle _Blaps mucronata_. _E. porrigens_ Rud. is found in considerable numbers in the small intestine of a fin-whale (_Balaenoptera sibbaldii_), and _E. strumosus_ Rud., in the small intestine of a seal (_Phoca vitulina_), and in the body-cavity of the angler fish (_Lophius piscatorius_). _E. acus_ is common in the whiting, _Gadus merlangus_.

FAMILY II. GIGANTORHYNCHIDAE.—Large forms with ringed, flattened, and _Taenia_-like bodies. The hook-papillae are covered all over with transparent chitinous sheaths with two root-like processes. The proboscis-sheath is muscular and without a lumen. The central nervous system is excentrically placed below the middle of the so-called sheath. The lemnisci are long twisted tubes with a central canal.

{184}Hamann places three species in this family: _Gigantorhynchus echinodiscus_, _G. spira_, and _G. taenioides_; but as he points out that _E. gigas_ resembles these in its more important structural features, it seems advisable to include it here under the name _G. gigas_. The members of the first family often present a transversely ringed appearance after death, but the Gigantorhynchidae are ringed when alive, and the circular canals in the skin show a certain regularity, being arranged one between each two rings. There is no lumen in the proboscis-sheath, which is not attached to the boundary between the proboscis and the trunk, but to the inner surface of the proboscis, and the whole can be retracted within the anterior portion of the body, which is invaginable. There are always eight cement-glands, and other differences exist in the musculature, hooks, and position of the nervous system.

_G. gigas_ occurs in the adult state in the small intestine of swine; in Europe its first or larval host is believed to be the grubs of _Melolontha vulgaris_ and _Cetonia aurata_, but these beetles are absent from America, though the parasite infests American hogs. Stiles[220] has recently made some experiments which tend to show that in the United States the source of infection is some species of the beetle _Lachnosterna_, and he has succeeded in infecting the grub of _L. arcuata_ by feeding it on the eggs of the parasite; from one larva he took 300 parasites six weeks after feeding it. _L. arcuata_ is, like _M. vulgaris_, phytophagous, but the grubs of both the beetles are fond of frequenting manure heaps and patches of dung, and thus are much exposed to the dangers of infection.

_G. echinodiscus_ inhabits the intestine of ant-eaters, having been found in _Myrmecophaga jubata_ and _Cycloturus didactylus_. _G. spira_ lives in the king vulture _Sarcorhampus papa_, and _G. taenioides_ in _Dicholophus cristatus_, a species of Cariama.

FAMILY III. NEORHYNCHIDAE.—Sexual maturity is reached in the larval stage. The proboscis-sheath has a single wall. A few giant nuclei only are found in the sub-cuticle and in the lemnisci. The circular muscle layer is very simply developed. The longitudinal muscle-cells are only present in certain places.

This family includes two species, _Neorhynchus clavaeceps_ and _N. agilis_, which afford interesting examples of paedogenesis. The sub-cuticle and the lemnisci are dominated by a few giant {185}nuclei, which remain in the embryonic state and do not break up into numerous nuclei as in other forms. The musculature is but little developed and the longitudinal sheath hardly exists. The proboscis-sheath consists of a simple muscular layer, and the short proboscis has few hooks and presents an embryonic appearance.

The sexually-mature form lives in the carp, _Cyprinus carpio_; the larval form is found, according to Villot,[221] encysted in the fat bodies of the larva of _Sialis lutaria_, one of the Neuroptera, and in the alimentary canal of the leech _Nephelis octocula_, and successful experiments have been made in infecting some species of the water snail _Limnaea_. _N. agilis_ occurs in _Mugil auratus_ and _M. cephalus_.

FAMILY IV. ARHYNCHIDAE.—Short forms with the body divided into three well-marked regions—head, collar, and trunk. The head is pitted, the collar smooth, and the trunk wrinkled, not annulated, in spirit specimens. There is no eversible introvert, and no introvert sheath and no hooks. The sub-cuticle and the lemnisci have a few giant nuclei, and the lemnisci are long and coiled.[222]

This family resembles the Gigantorhynchidae in the length and curvature of its lemnisci, and the Neorhynchidae in the persistence of the embryonic condition of the nuclei in the sub-cuticle and the lemnisci; but in the shape of the body, its division into three well-marked regions, the absence of eversible proboscis, proboscis sheath, and hooks it stands alone, though it is nearer to the Neorhynchidae than to either of the other families.

The single species _Arhynchus hemignathi_ was found attached to the skin around the anus of a Sandwich Island bird, _Hemignathus proceros_. The bird is a member of a family Drepanididae, which is entirely confined to the Sandwich Island group. Professor Newton tells me that it is probable that the "food of _Hemignathus_ consists entirely of insects which it finds in or under the bark of trees," hence it is probable that the second host of this parasite, if such exists, must be looked for amongst the Insecta.

{186}CHAPTER VII

CHAETOGNATHA

STRUCTURE—REPRODUCTION—HABITS—FOOD—CLASSIFICATION TABLE OF IDENTIFICATION

At certain seasons and at certain times of the day the naturalist who is investigating the fauna of the surface of the sea is apt to find his tow-net crammed with innumerable transparent spindle-shaped animals, which by their number and the way in which they become entangled with rarer objects, often render useless the result of his labours. These animals belong to the class Chaetognatha, which includes three genera, _Sagitta_, _Spadella_, and _Krohnia_. Amongst them are divided about twenty species, some of which, however, are of doubtful value.

ANATOMY.—The body of these animals is as transparent as crystal; it is elongated, and bears a resemblance to certain torpedos, except that the head forms a somewhat blunt termination to the spindle-shaped body. The tail bears a caudal fin, and _Spadella_ and _Krohnia_ have a single pair, and _Sagitta_ two pairs, of lateral fins; all of which are flattened horizontally.

The body is externally divisible into three regions—head, trunk, and tail—and these correspond with the arrangement of the internal organs.

The head is surrounded by a fold of skin, forming a hood, {187}which is most prominent at the sides (Fig. 102, _g_); within the hood the head bears from two to four rows of short spines, and outside these a right and left row of sickle-shaped hooks, the free ends of which in a state of rest converge round the mouth, but when disturbed these hooks can be widely divaricated.

The cavity of the body, or coelom, is divided into three distinct chambers by the presence of two thin transverse walls or septa, one situated between the head and the trunk, the other between the trunk and the tail (Figs. 104, 105). In the head, this cavity is much reduced by the presence of special muscles which move the spines, hooks, etc.; and in the small species, such as _Spadella cephaloptera_, the other two cavities are almost entirely occupied by the digestive and reproductive organs[223]; but in the large species, e.g. _Sagitta hexaptera_, a considerable space is left between the internal organs and the skin, and this is occupied by a coelomic fluid. If the skin of one of these larger species be punctured the fluid escapes and the animal shrivels up. A longitudinal partition or mesentery, with numerous pores in it, runs through these spaces, dividing the body-cavity into a right and left half; in the region of the trunk this mesentery supports the alimentary canal.

In addition to certain muscles in the head, which move the hooks, etc., there is a muscular lining to the body-wall. This is divided into two dorsal and two ventral bands, much in the same way as in Nematodes. The muscle fibres are striated.

The mouth, situated either terminally—_Spadella marioni_[224]—or below the head, leads into a pharynx; this passes into an intestine lined by a single layer of ciliated cells with a few glandular ones intermingled. The intestine runs straight through the body without loop or coil, and opens by an anus situated at the junction of the trunk and the tail. In most cases the anus is ventral or on the lower surface, but Gourret asserts that in _Spadella marioni_ it is on the upper surface.

There are no special respiratory, excretory, or circulatory organs, unless a glandular structure described by Gourret in the head of _Spadella marioni_ be a real kidney.

The nervous system consists of a supra-oesophageal ganglion {188}or brain situated in the head, and of a ventral ganglion lying in the trunk; both these nerve centres are embedded in the epidermis, and are connected with one another by means of two stout peri-oesophageal nerves (Figs. 102, 104). The brain also gives off a pair of nerves to the eyes, another pair to the olfactory organ, and a pair which ultimately meet one another and so form a ring; on this are certain ganglia giving off nerves which supply the muscles of the head. Both the chief ganglia give off numerous nerves, which divide and split up into a network of fibres which permeate the whole skin.

The sense organs are comparatively simple. A pair of very small eyes lie in the skin of the head; they are of complex structure, and to some extent remind one of the simple eyes of certain Crustacea. Behind the eyes and also on the upper surface of the animal is an unpaired organ which is usually described as olfactory in function (Figs. 103, 105). This is a ring-shaped modification of the epidermis drawn out into different shapes in the various species. The modified epidermal cells bear long cilia. The remaining sensory organs found in the group consist of clumps of modified cells scattered in round groups over the surface of the body and of the fins. The central cells of each group bear long tactile hairs, and are surrounded by supporting cells.

The Chaetognatha are hermaphrodite, and carry the female organs in the trunk, the male in the tail. In a mature specimen the two ovaries occupy almost all the space in the trunk between the alimentary canal and the skin, and each is supported by a narrow lateral mesentery. The ovary is traversed by a oviduct which often contains spermatozoa; it is not clear how the eggs make their way into the oviduct, which seems to have {189}no internal opening and to act largely as a receptaculum seminis. The oviducts open externally on the upper side at the base of the lateral fin, close to the junction of the tail and the trunk.

The cavity of the tail is divided into two lateral chambers by the extension backward of the median vertical mesentery. In each of these a testis and a vas deferens are found. The testes are solid ridges formed by the growth of the lining cells of this part of the body-cavity; the cells mature into spermatozoa, which break off and float freely in the coelomic fluid. At the breeding season the whole tail may be crowded with masses of spermatozoa, which are kept in a more or less regular circulation by the ciliated cells lining the body-wall. The vas deferens opens internally into the space where the spermatozoa lie, and at the other end into a vesicula seminis, which opens to the exterior. The position of the latter structure varies, and is of some systematic value.

The eggs are laid in the water and as a rule float at the surface of the sea. _Spadella cephaloptera_ is, however, an exception to this rule, as it attaches its eggs by means of a gelatinous stalk to sea-weeds. The segmentation of the ovum is regular, and gives rise to a two-layered stage or _gastrula_, which opens by a pore, the _blastopore_. This does not, however, become the mouth, but closes up and the mouth arises at the opposite pole. Perhaps the most interesting feature of the development of _Sagitta_ is that the cells destined to form the reproductive organs separate from the other cells of the embryo at a very early date, whilst it is still in the gastrula stage. There is no larval form, but the young hatch out from the egg in a state resembling the adult in all respects but that of size.

HABITS.—The Chaetognatha are essentially pelagic, and {190}resemble many other creatures that dwell at the surface of the ocean in being almost completely transparent. Most species have been taken far out at sea, but some are perhaps rather more numerous near the coast, and one species, _Spadella cephaloptera_, is littoral. They swim by means of muscular movements of the whole body; the fins have no movement of their own, and seem to serve as balancers, and not as locomotory organs. Although usually found at the surface of the water, many species have been taken at considerable depths. Chun[225] states that they are found in countless numbers at depths of from 100 metres to 1300 metres. The commonest species at these depths are _Sagitta hexaptera_ and _Sagitta serratodentata_. _Sagitta bipunctata_, according to the same authority, confines itself to the surface. Whether the change of depth is diurnal, or whether it has any relation to sexual maturity, or to any other cause, has not been satisfactorily determined.

The food of the Chaetognatha consists of floating diatoms, Infusoria, small larvae, and such Copepods as _Calanus finmarchicus_, and small Amphipods as _Phoxus plumosus_.[226] At times they also devour small larval or post-larval fishes, and owing to their incredible numbers, they doubtless do considerable damage to sea fisheries. It is also recorded that they eat one another, and specimens have been taken which have ingested the whole body of another _Sagitta_ except the head, which hangs out of the mouth of the eater, and gives it the appearance of a double-headed monster.[227] It has been said that they attack hydroid polypes, but here at any rate they do not have it all their own way. Masterman[228] has figured the apical group of five polypes of _Obelia_, three of which are engaged in ingesting as many young _Sagitta_.

They exist in incredible numbers; Grassi describes the surface of the sea at Messina on certain days as being literally covered with them, and they must form the food supply of numerous animals which prey upon the pelagic fauna. The immense number of individuals is probably accounted for to some extent by the fact that they lay eggs all the year round, and pass {191}through a very short and rapid development. They are not known to be phosphorescent.

CLASSIFICATION.—The features of the Chaetognatha which have most systematic value are the size of the adult, the relations of the length to the breadth, and of the three divisions to one another; the size, number, and position of the lateral fins, and of the hooks and spines on the head; the thickness of the epidermis, and the structure of the olfactory organ; and, finally, the form of the reproductive organs.

Strodtmann,[229] who gives the latest and most complete account of the species of Chaetognatha, arranges them under three genera, which he characterises as follows:—

(i.) _Sagitta_ Slabber.—Two pairs of lateral fins, two rows of spines on the head. The lateral thickening of the epidermis absent or insignificant.

Under this genus are included nine definite species and five others—_S. gracilis_ Verrill, _S. elegans_ Verrill, _S. darwini_ Grassi, _S. diptera_ d'Orbigny, and _S. triptera_ d'Orbigny—whose position, owing to the inadequacy of their description, is of doubtful validity.

The distribution of the other species may be mentioned. _S. hexaptera_ is the largest Chaetognath known, and reaches in the adult stage a length of 7 cm. It is very widely distributed, being found in practically all the temperate and warm seas, usually at the surface of the water, though at times it is found at a depth of one metre, or even deeper. _S. lyra_, Mediterranean, very rare. _S. tricuspidata_, widely distributed. _S. magna_, Mediterranean and Madeiran, living at the surface. _S. bipunctata_, the most frequently described form, smaller than the preceding species, 1-2 {192}cm. in length, widely distributed, and as a rule living near the coast line. _S. serratodentata_, Mediterranean. _S. enflata_, on the surface of the sea, Mediterranean and Madeiran. _S. minima_, a very small species, 1 cm. in length, Mediterranean. _S. falcidens_, Atlantic, off the coast of New Jersey.

(ii.) _Krohnia_ Langerhans.—A single lateral fin extending on to both trunk and tail segment, no lateral epidermal extensions behind the head, only one row of spines on the head. Trunk longer than the tail.

_Krohnia_ has but two species: _K. hamata_ Möbius, with a length of 3-4 cm., found in the North Atlantic and at considerable depths, 200 to 300 fathoms; and _K. subtilis_ Grassi, 1.5 cm. long, with an extraordinary slender body and a relatively large head, found at Messina, but very rare; as a rule only one specimen has been found at a time.

(iii.) _Spadella_ Langerhans.—A single pair of lateral fins; these are situated on the tail segment. Behind the head a thickening of the epidermis extends down each side of the body to the fin, or even farther. Two rows of spines on the head. Small animals, not longer than 1 cm.

_S. cephaloptera_ Busch is the smallest species of Chaetognatha, attaining at most a length of .5 cm. The body is not so transparent as in other species, and is of a yellowish colour. It has been found from the Orkney Islands to the Mediterranean. Strodtmann is of the opinion that the three species _S. mariana_ Lewes, _S. batziana_ Giard, and _S. gallica_ Pagenstecher differ from the above-named only in size, or that their description is too indefinite to permit of accurate {193}characterisation. He recognises three other distinct species: _S. pontica_ Uljanin, from the Black Sea; _S. marioni_ Gourret, from the Gulf of Lyons; and _S. draco_ Krohn, Mediterranean and Madeiran, and from the Canaries.

Much confusion has been introduced into the classification of the Chaetognatha by Grassi,[230] who calls some—but not all—of what other writers term _Sagitta_, _Spadella_, and _vice versâ_. The following table was compiled by Strodtmann,[231] but I have incorporated in it two species recently described from Amboyna by Béraneck,[232] and called by him _Sagitta bedoti_ and _Spadella vougai_ respectively:—

CHAETOGNATHA

I. Two pairs of lateral fins; two rows of spines on the head; slender
forms.

(i.) Number of spines in posterior row greater than in anterior.

_a._ Border of hooks smooth, their point not curved.

α. No interval between the two fins on each side. 3.5 cm. long; 4-7
anterior spines, 8-11 posterior spines; olfactory organ lying
entirely on the trunk. The anterior nerves of the ventral ganglion
lie close to one another as far as the head.—SAGITTA LYRA.

β. A distinct interval between the two fins on each side.

_aa._ Adult animals large; hooks 6-7; anterior spines 3-4;
posterior spines 5-7; tail ¼ or ⅕ of the total length; lateral
areas relatively larger.—SAGITTA HEXAPTERA.

_bb._ Greatest length 1-2 cm.

αα. Thickening of the epidermis behind the head; prominently
projecting vesiculae seminales; olfactory organ very long;
hooks 8-10; anterior spines 4-6; posterior spines
10-15.—SAGITTA BIPUNCTATA.

ββ. No epidermal thickening; two caeca on the anterior end of
intestine; length 1 cm.; hooks 6-9; anterior spines 3-4;
posterior spines 7-8; point of the hooks somewhat bent
round.—SAGITTA MINIMA.

γγ. Epidermis thin; no caeca; hooks 8-9, their ends not bent;
anterior spines 3-4; posterior spines 7-8; length 2 cm.; small
head; trunk proportionately thick.—SAGITTA ENFLATA.

δδ. Hooks 11-14, usually 12; length 1.8 cm.; anterior spines
6-7; posterior spines 18.—SAGITTA FALCIDENS.

{194}εε. Hooks 7 on each side; length 1.3 cm.; anterior spines
8-10, posterior spines 18-22; no olfactory organ.—SAGITTA
BEDOTI.

_b._ Edge of hooks toothed and their point bent round; hooks 6-8;
anterior spines 6-8; posterior spines 10-12; length 1.5 cm.; slender;
conspicuously projecting vesiculae seminales.—SAGITTA SERRATODENTATA.

(ii.) Number of the spines in posterior row smaller than in anterior.

_a._ Anterior spines 3; posterior spine 1; hooks 8; length 3.5
cm.—SAGITTA TRICUSPIDATA.

_b._ Anterior spines 4; posterior spines 3; hooks 10-13; length 4.1
cm.; tail ⅕ of the total length.—SAGITTA MAGNA.

II. One pair of lateral fins lying on the trunk and tail; one row of
spines; body slender; epidermis not thickened.

(i.) Hooks 8-9, bent like an elbow at the point, serrated in the young;
20-25 spines in a row; ovary reddish; length 3-4 cm.—KROHNIA HAMATA.

(ii.) Hooks 8, broad at their base but very sharply pointed; spines in
a curved row, about 18, with a constriction below like the neck of a
bottle; body thin; length 1-1.5 cm.—KROHNIA SUBTILIS.

III. One pair of lateral fins, these lie on the tail; body relatively
very broad in consequence of the thickening of the epidermis lying behind
the head; two rows of spines; greatest length 1 cm.; tail and trunk
usually the same length.

(i.) A great extension of the epidermis behind the head, consisting of
very large cells; amongst these, at the level of the ventral ganglion,
lies a bundle of stiff hairs; tactile organ on papillae; hooks 9-10;
anterior spines 6-8; posterior spines 12-18.—SPADELLA DRACO.

(ii.) Lateral extension of the epidermis not so conspicuous, and the
cells composing it smaller. Tactile organs in little depressions.
Transverse as well as longitudinal muscles in the trunk. Adhesive cells
on the ventral surface of the body. No interval between the lateral
fins and the tail fin. Two papillae on the head-hood elongated into
club-shaped tentacles. Hooks 8-9, slightly serrated; anterior spines
3-4; posterior spines 3-4.—SPADELLA CEPHALOPTERA.

(iii.) Similar to the last-mentioned species, but the tail segment is
larger than the trunk; in the above it is of the same size. No adhesive
cells. The fins are covered with papillae, and with a number of
serrated spines pointed at both ends.—SPADELLA PONTICA.

(iv.) Tactile organs and adhesive cells are unmodified epidermal cells.
Anus dorsal. Orifice of oviducts ventral. No olfactory organ. Epidermis
colourless. Lateral fins without rays. A pair of ganglia at the
postero-lateral angle of the brain.—SPADELLA MARIONI.

(v.) Tactile organs well developed on the head, trunk, and fins; tail
segment a little shorter than the trunk. Body short, length 3-4 mm.
Hooks 9; anterior spines 4-5, posterior spines 6-7.—SPADELLA VOUGAI.

ROTIFERA, GASTROTRICHA, AND KINORHYNCHA

BY

MARCUS HARTOG, M.A., Trinity College (D.Sc. Lond.)

Professor of Natural History in the Queen's College, Cork.

{197}CHAPTER VIII

ROTIFERA, GASTROTRICHA, AND KINORHYNCHA

ROTIFERA—HISTORY—EXTERNAL FEATURES—MOVEMENT—ANATOMY—REPRODUCTION— EMBRYOLOGY—CLASSIFICATION—DISTRIBUTION—AFFINITIES—GASTROTRICHA— KINORHYNCHA

The Rotifera are microscopic animals, the largest not exceeding one-eighth of an inch in length. According to Hudson and Gosse,[233] they are first recorded in an observation of the Rev. John Harris, in 1696, of "an Animal like a large Maggot which could contract itself into a Spherical Figure, and then stretch itself out again; the end of its Tail appeared with a _Forceps_ like that of an _Ear-wig_."[234] This was certainly a Bdelloid Rotifer.

In 1703 Leeuwenhoek[235] gave a fuller description of a tubicolous form, probably _Limnias_, and noted the peculiar appearance of the ciliary wreath as "two wheels thickset with teeth as the wheel of a watch." He also noted a little later[236] the way in which _Melicerta_ (see p. 206) builds its tube, and was the first to observe the revivification of certain species after drying.[237] Joblot, a French professor of mathematics, in 1718 figured and described a large number of new genera and species with more or less fantastic details. Baker's figures[238] are a considerable advance on Joblot's, and his descriptions of habits are still fresh and accurate. Eichhorn found a number of new and interesting forms; and O. F. Müller, influenced by the new discipline of Linnaeus, not {198}only figured many species, but gave good short diagnoses of their characters. Ehrenberg in 1838 brought out his magnificent _Infusionsthierchen_, which contains descriptions and figures of what are now divided into Protophyta, Protozoa, Rotifera, and Gastrotricha. Dujardin's monograph on the "Infusoires," in the _Suites à Buffon_,[239] was in several respects an advance on Ehrenberg, whose power of observation was so great as to render his mistakes the more inexplicable. But Ehrenberg ever adhered to his errors as firmly as to his facts.

The occurrence of Rotifers among microscopic plants induced the botanists Cohn and Williamson[240] to work at their structure; the group has been studied by men engrossed in other professional cares, such as Gosse, Bedwell, Moxon, Rousselet, and Maupas. Huxley,[241] Leydig,[242] and Cohn[243] studied Rotifers in the '50's and early '60's with a precision the more remarkable when we remember the imperfect methods then available. This period was closed by the valuable monograph published in Arlidge's (4th) edition of Pritchard's _Infusoria_,[244] under the supervision of W. C. Williamson. Leidy began the study of the American Rotifers. Eckstein[245] gave a careful and interesting account of the species about Giessen in a richly illustrated paper. In recent times the modern methods of histological and embryological research have been applied by Vallentin,[246] Plate,[247] Tessin,[248] and Zelinka,[249] the three _Studien ueber Rotatorien_ of the last author being indispensable to every student, and containing a full bibliography.

Hudson and Gosse's Monograph (1886-89) contains a history of the class to which, as to the whole book, we are deeply indebted; and a full systematic account of all published species.[250] C. Rousselet has introduced a method[251] of preparation of Rotifers in microscopic slides which enables workers to preserve the types they figure and describe for future identification and comparison. Gunson Thorpe has collected and studied Rotifera in China and {199}Australia. It would be unfair not to record here the invaluable services of the late Thomas Bolton, and his son of the same name, both of Birmingham, and of J. Hood of Dundee, who have found and widely distributed living specimens of new, rare, and interesting species.

DEFINITION OF THE CLASS.—We may define Rotifera as a class of minute bilaterally symmetrical animals, with a chitinous integument, a soft terminal "disc" fringed by a complex ciliary "wreath," an anterior or subventral mouth, and a dorsal cloacal aperture, beyond which the body is usually prolonged into the "foot" or process bearing cement glands, and serving for attachment, temporary or permanent. The body-cavity has no epithelial lining, and is traversed by nerves and muscles. The alimentary canal possesses a chitinous gizzard or _mastax_ of peculiar arrangement, and it usually opens into a cloaca. The nervous centre consists of a ganglion on the dorsal side of the pharynx, to which a second one on the ventral side is sometimes connected to form a complete ring; eyes and bristle-bearing feelers are usually present as sense-organs. A paired system of renal tubes serves for excretion, opening through a median contractile bladder into the ventral side of the cloaca. The sexes are distinct; but the males (Fig. 107), which mostly lack digestive organs, occur {200}rarely, and the females are usually viviparous, or carry about the eggs till they are hatched; while, owing to the rarity of the males, parthenogenesis is habitual. Fission and budding are alike unknown. The fertilised eggs are of the kind termed "winter" or "resting" eggs, and resist conditions adverse to life.

The Rotifera are of cosmopolitan distribution; most of the species inhabit fresh water, whilst some are brackish, and a few are marine; 84 genera and about 700 species have been described.

) 1, _Floscularia campanulata_; 2, _Lacinularia socialis_; 3, _Notops brachionus_; 4, _Synchaeta tremula_; 5, _Asplanchna ebbesbornii_; 6, _Brachionus urceolaris_; 7, _Salpina mucronata_; 8, _Pedalion mirum_.]

EXTERNAL FEATURES.[253]—The body is divided into three regions: (1) the _head_, ending in the _disc_, which bears the ciliary _wreath_; (2) the _trunk_, containing the viscera; (3) the _foot_, which only contains muscles, nerves, and cement-glands. The general form of the BODY varies greatly: it is spherical in _Trochosphaera_, ovoid in Asplanchnidae, conical in Scirtopoda, Triarthridae, and _Synchaeta_; moderately elongated in the majority of the Ploima, among which some forms are very flat, like _Pterodina_, _Metopidia_, and _Brachionus_; shortly elongated and cylindrical in _Hydatina_ (Fig. 106), Notommatidae, and many others. In _Taphrocampa_ it is cylindrical and segmented, while the segments are {201}telescopic in the Bdelloida, both ends being retractile into the middle segment. In most attached, tube-dwelling forms the body is ovate, tapering behind into the elongated stalk-like foot.

The FOOT at the hinder end of the body is usually more or less jointed; in _Pterodina_ and _Brachionus_ it is long, transversely wrinkled, and retractile. Usually it terminates in a couple of acute, mobile toes, perforated at the tips by the ducts of the pedal glands (Fig. 106, _fg_), whose viscid secretion serves to anchor the animal. In _Rotifer_ there are three of these toes, which are retractile, and in addition there are in this genus, as in most of the Bdelloida, toe-like pointed spurs in pairs on the more proximal joints of the foot. In _Callidina_ the spurs are often perforated, and the toes are replaced by numerous openings on the last joint of the foot (Fig. 109, A); while in _Discopus_ the end of the foot expands into a large disc, with numerous pores for the exudation of the pedal cement, and there are no spurs. In _Pedalion mirum_ the foot is represented by two tubular processes ciliated at the apex and at the outer side near the base (Fig. 117, _f_). These are inconstant in size and form, that of one side being sometimes reduced or absent, while both are absent in the closely allied species _P. fennicum_.

In Melicertidae and Flosculariidae the long foot ends in an expanded disc, which is cupped and ciliated in the larva (Fig. 112, B) and in the larva-like male (Fig. 107); but in two species it is prolonged into a long flexible thread which is not contractile. The foot is also elongated in the Bdelloid genus _Actinurus_ and the Ploimal genus _Scaridium_. It forms a mere ventral disc in _Apsilus_ (and _Atrochus?_), and is absent in Asplanchnidae (except _Asplanchnopus_), Triarthridae, and Anuraeidae, and in the genera _Trochosphaera_ (Melicertaceae) and _Pompholyx_ (Pterodinidae).

The fringed spines of Triarthridae are jointed appendages moved by powerful muscles; in _Triarthra_ one is median and ventral, the others being attached to the shoulders. In _Polyarthra_, there are twelve flattened and serrated spines, a bunch of three being attached to the dorsal and ventral faces of either shoulder. An easy transition leads to the hollow appendages of Scirtopoda, which end in a fringe of bristly hairs, themselves feathered with finer hairs (Fig. 117). These processes are in _Pedalion_ six in number, two median (respectively dorsal and ventral), two antero-lateral, and two postero-lateral. As they contain proper muscles, {202}and the postero-lateral pair contain part of the nephridia and bear the lateral antennae, they are true outgrowths of the body, and are not homologous with the spines of Triarthridae.

The front of the body constitutes the HEAD, which is scarcely distinct, though usually separated by a slight neck-like constriction. The DISC, which terminates the head, varies greatly in shape and in the arrangement of its parts. Imagine a circular funnel, finely ciliated within, and with the mouth at the bottom, the prominent rim bearing two zones of cilia, the inner or anterior being the coarser, and termed the "trochus" or _hoop_; the outer finer, and termed the "cingulum" or _girdle_, while a very finely ciliated groove lies between the two zones. Either or both of these zones may be interrupted on the dorsal or ventral median line, or both; and the funnel-shaped mouth may be shifted—usually ventrally, so that it forms only a dilatation of the ciliated groove. Again, the wreath as a whole may be festooned or lobed; or the lobing may be confined to the area between the cingulum and trochus, as in most Ploima (Figs. 106 and 108, 3). Very frequently on these lobes adjacent cilia are fused together during life, producing "vibratile styles," whose true nature is only revealed after death. In Microcodonidae the structure of the disc (Fig. 108, 1) nearly conforms to the primitive type; but the ciliated groove is absent, and the "trochus" is in two separate half-elliptical bands. In the Flosculariaceae (Fig. 108, 6) the mouth is also central, the disc is funnel-shaped, {203}and the trochus is a horseshoe-shaped ridge, with its ends dorsal and raised into prominent knobs. The margin of the funnel is in Flosculariidae (Fig. 115) usually lobed, and furnished either with exceptionally strong cilia, or else with very long bristles which are usually passive. However, by the retraction of the lobes that bear them they are clasped together like casting-nets to enclose prey brought into the funnel by the action of the trochal cilia. An external ring of cilia in _Floscularia mutabilis_ and _F. pelagica_ serves for swimming. In Apsilidae the margin of the disc bears neither cilia nor bristles, but is either simple and ring-like, or is produced into tentacles (Fig. 112, C). The oral funnel is probably represented in Flosculariaceae by the continuation of the small central mouth into a ciliated tube (Fig. 115, C, _tf_), open below, and hanging freely down into the crop.

In all other cases the mouth is displaced, and lies in the groove and on its ventral side (except in _Conochilus_, where it is dorsal, Fig. 108, 5). In the Bdelloida the disc is prolonged into two great lobes like kettle-drums, round the posterior, external, and ventral edges of which run the trochus, cingulum, and ciliated groove (Fig. 108, 2). All three are interrupted behind in the median line; ventrally the groove widens into the oral funnel, the cingulum is continued into a sort of spout-like lower lip (Fig. 109, C, D, _l_), and the trochus is absent. The body is prolonged dorsally above the lobes into a two-jointed _proboscis_, ending in a ciliated cup overhung by two dorsal flaps: this we regard as a detached portion of the wreath.

This "Bdelloid" type of wreath occurs also in Scirtopoda (Fig. 117), and in the Ploimal genera _Triarthra_, _Pterodina_, and _Pompholyx_. A simpler wreath of essentially the same type occurs in Asplanchnaceae and Melicertaceae; the disc is not prolonged into drum-shaped lobes, but is thin at the rim, where it bears the triple ciliated zone, interrupted on the dorsal median line and depressed ventrally into the oral funnel. In the Melicertidae, moreover, the disc is widened into a great plate-like extension, often beautifully lobed; and in many of the species a ciliated cup lies ventral to the lips, and is connected with the groove by a short ciliated channel on either side (Figs. 108, 4, and 116). Even the simpler wreath of Asplanchnidae is complicated by stronger lobes on either side bearing vibratile styles.

The most complex discs are found in Ploima, especially in {204}_Brachionus_, _Hydatina_, and _Synchaeta_, since the groove is replaced by a zone of lappets, as above mentioned. In _Proales_ the whole face of the disc is strongly ciliated. The wreath is reduced in the parasitic genera _Drilophagus_, _Albertia_, _Balatro_, and the Seisonaceae; in _Adineta_ and _Taphrocampa_ it is only represented by a general but scanty ciliation of the disc.

The head is very frequently retractile, as a whole, by strong muscles. In Bdelloida the disc proper is retracted when the animal crawls, while the proboscis is exserted (Fig. 109). Ciliated patches occurring outside the region of the disc point to {205}a primitive condition when the whole surface of the body was ciliated, as does the partial ciliation of the foot in certain groups. _Synchaeta_ and many Notommatidae possess a pair of lateral, hollow, ciliated pits on the body, which can be everted to serve as additional swimming organs; these are termed "auricles."

The _cuticle_ varies much in texture. It may be smooth and flexible, dotted or shagreened, or in the Loricata firm and of definite shape, constituting a _lorica_, which may be more or less distinctly divided up into areas or separated into distinct pieces. In this case it resists decomposition, and several species are only known by this "skeleton." In _Ploesoma_ it is much thickened and looks like a honeycomb. A regular alternation of harder and softer zones effects the annulation of the body in certain genera.

The _hypoderm_ or protoplasmic layer of the skin has no cellular boundaries, though it contains large and distinct nuclei; it is usually somewhat granular. It forms the wall of the body-cavity, which contains a transparent liquid without corpuscles.

The principal _external glands_ are the pedal or cement-glands, which secrete a viscid substance that sets in water and serves to anchor the animal. They are formed from an ingrowth of the hypoderm, are usually paired, and open by fine ducts on or near the apex of the toes, when these processes of the foot are present (Fig. 106, _fg_). These glands are mostly absent when there is no foot, as in most Asplanchnidae and in Anuraeidae, but in _Asplanchna herrickii_ a small gland on the ventral side of the cloacal aperture appears to represent the last rudiment of the foot.

In addition to these, the ciliated ventral cup below the disc of many Melicertidae secretes a viscid substance (Fig. 116, _p_); and possibly the whole surface of the body is secretory in those species of this group, and of the Flosculariidae, whose tube (Fig. 115, A) is uniform and not made of pellets. In several other species belonging to Bdelloida and Ploima-Illoricata a viscid secretion of the surface of the body renders it "sordid" with adherent particles of dirt.

When the secretion takes the form of a tube, the body can be wholly withdrawn into it by the contraction of the foot. In _Floscularia_, _Stephanoceros_, and _Conochilus_ the tube is hyaline and thin-walled; in _Oecistes_ and _Cephalosiphon_ it is more or less floccose; and in _Limnias_ it is thin, firm, and annulated. In _Melicerta_ and some species of _Oecistes_ the tube thus secreted by {206}the body is only formed in a very young state. In _M. janus_ and _M. pilula_ it is increased by the successive deposition of ovoid faecal pellets on to the rim. In _M. ringens_ (Fig. 116) and _M. conifera_ pellets are formed of the excess of the food particles brought to the disc by the ciliary current; they are carried through the gutters on either side of the projecting ventral lip or "chin" into the ciliated glandular cup on that side of the head. Here, as they revolve, they are cemented together into a pellet which is spheroidal in the former species, cylindro-conoidal with a basal hollow like a rifle-bullet in the latter. After a pellet is completed the animal stoops down and deposits it on the edge of the tube. This may easily be verified by furnishing a young _Melicerta_ with water containing solid particles of carmine. _M. tubicolaria_ forms a thick tube which is laminated, the laminae being directed upwards and outwards, and having diatom shells, etc., between the layers. In this case we have observed that the faeces are pellucid, and sometimes are so ejected as to lie in a sheet against the funnel-shaped mouth of the tube, and we are inclined to believe that the tube itself is formed altogether in this way. A similar process probably occurs in _Oecistes crystallinus_ and _Oe. umbella_.

The _muscles_ are simple elongated fibres, usually having near the middle a mass of granular protoplasm containing a nucleus; they may be smooth or striated. The principal muscles of the body are conspicuously striated in many active free-swimming forms (_Pedalion_, _Synchaeta_, _Pterodina_, _Triarthra_).

The muscles of the body-wall are transverse and longitudinal. They are best seen in Bdelloida. The principal muscles of the body-cavity are longitudinal; the most conspicuous and constant are the retractors of the disc and of the foot, protraction of these organs being usually accomplished by the contraction of the transverse muscles. Special muscles effect the vigorous springing of the Triarthridae and Scirtopoda; in the former group the muscles only raise the spines, and their elastic recoil is the actual mechanism of progression; but in the latter (Fig. 117) special flexor muscles of the limbs are the effective agents of the leaping movements.

MOVEMENTS.—The Rotifera vary very greatly in their movements. The cilia of the disc, and especially of the trochus, are the principal organs of prehension of food, and also of swimming when {207}the animal is not fixed by its foot. In some cases, as in Bdelloida, the cilia lash downwards successively in the longitudinal plane of the body (Fig. 109, C, D); this motion during fixation produces a hollow vortex ring, like the rings of a skilled cigarette-smoker, but when the animal is free it determines a simple forward progression through the water. In other cases the animal rotates on its long axis, or may even turn somersaults (_Synchaeta_). The appearance of the spokes of a wheel is a pure illusion due to the greater visibility of the cilia in their slow recovery than in their instantaneous down-lash. The finer cilia of the groove and cingulum play a very minor part in the act of swimming, and in the production of the great vortices at the edge of the disc when the animal is fixed; they serve to direct the particles brought by the vortices to the edge of the disc onwards towards the mouth. It is easy to see that the stream must be in opposite directions on opposite sides of the groove; its prolongation across the dorsal median line would be useless, which explains the existence of the dorsal median gap. At the ventral side we usually find a prominent ciliated lip, whose cilia work outwards, and carry off the excess of food particles as by an overflow spout. In many cases among the Notommatidae, Coluridae, etc., the disc serves as much for creeping over organic débris as for swimming.

We have already noticed the springing bristles and limbs of the Triarthridae and Scirtopoda respectively; the great foot of _Scaridium_ is also used for leaping. The Bdelloida have the power of retracting their disc and progressing in loops like a leech or looper (Geometrid) caterpillar.

Baker, in a letter addressed to Martin Folkes, Esq., President of the Royal Society, dated London, 16th January 1744-5,[254] gives the following lively account of the aspect and movements of _Philodina roseola_ belonging to this group, with figures, some of which we reproduce from the original copper-plate engraving:—"I call it a _Water Animal_, because its Appearance as a living Creature is only in that Element. I give it also for Distinction Sake the Name of _Wheeler_, _Wheel Insect_ or _Animal_; from its being furnished with a Pair of Instruments, which in Figure and Motion appear much to resemble Wheels. It can, however, continue many Months out of Water, and dry as Dust; in which Condition its Shape is globular, its Bigness exceeds not a Grain of {208}Sand, and no Signs of Life appear. Notwithstanding, being put into Water, in the Space of Half an Hour a languid Motion begins, the Globule turns itself about, lengthens by slow Degrees, becomes in the Form of a _lively Maggot_, and most commonly in a few Minutes afterwards puts out its Wheels, and swims vigorously through the Water in Search of Food; or else, fixing by its Tail, works them in such a Manner as to bring its Food to it. But sometimes it will remain a long While in the Maggot Form and not shew its Wheels at all....

"If the Water standing in Gutters of Lead, or the slimy Sediment it leaves behind, has any Thing of a red Colour, one may be almost certain of finding them therein,[255] and, if in Summer, when all the Water is dried away, and nothing but Dust remains, that Dust appears red, or of a dark brown, one shall seldom fail, {209}on putting it into Water, to discover Multitudes of minute reddish Globules, which are indeed the Animals, and will soon change their Appearance, in the Manner just now mentioned....

"A Couple of circular Bodies, armed with small Teeth like those of the Balance-Wheel of a Watch, appear projecting forwards beyond the Head, and extending sideways somewhat wider than the Diameter thereof. They have very much the Similitude of Wheels, and seem to turn round with a considerable Degree of Velocity, by which Means a pretty rapid Current of Water is brought from a great Distance to the very Mouth of the Creature, who is thereby supplied with many little Animalcules and various Particles of Matter that the Waters are furnished with.

"As these _Wheels_ (for so from their Appearance I shall beg Leave to call them) are every where excessively transparent, except about their circular Rim or Edge on which the Cogs or Teeth appear, it is very difficult to determine by what Contrivance they are turned about, or what their real Figure is, though they seem exactly to resemble Wheels moving round upon an Axis....

"As the Animal is capable of thrusting these Parts out, or drawing them in, somewhat in the Way that Snails do their Horns, the Figure of them is different in their several Degrees of Extension and Contraction, or according to their Position to the Eye of the Observer, whereby they not only appear in all the various Forms before represented, but seem at certain Times as if the circular Rim of the Wheel or Funnel were of some Thickness, and had two Rows of Cogs or Teeth, one above and the other below that Rim."

DIGESTIVE ORGANS.—The _pharynx_ is usually a narrow ciliated tube, which varies in length from genus to genus, but in no other important point, save in Flosculariidae, where it assumes the form of a crop, into which the mouth hangs freely down as a narrow ciliated tube. At its lower end is an enlargement, the _mastax_ or gizzard.[256] This is a strong muscular sac containing the _trophi_ or hard chitinous chewing organs, with an {210}antero-ventral inlet from the pharynx, and a postero-dorsal outlet through which the food passes into the stomach either directly or through a slender gullet (Fig. 106, _oe_). In the ventral wall of the gizzard of most Ploima is a median piece, the _fulcrum_, from which run forwards and upwards two pieces, the _rami_, which are hinged on the fulcrum. The Y-shaped structure formed of these three pieces is called the _incus_ (anvil). At either side of the gizzard and at a higher level is a paired piece, the _malleus_, so called from its resemblance to a hammer, of which the _manubrium_ (handle) looks backwards, and is embedded in the side walls of the mastax, while the toothed claw or _uncus_ looks forwards and inwards, and is hinged at its inner side with the tip of the ramus. As the unci and rami are usually strongly toothed, this gizzard forms a very efficient apparatus for chewing. In some cases, when the pharynx is short and dilatable, the points of the unci and rami may be protruded for biting, for clinging to the host (in the parasitic genera _Albertia_ and _Drilophagus_), or for the prehension of food (Rattulidae, etc.).

The type we have just described is termed the "malleate" type (Fig. 111, A). If all the trophi are slender and scarcely toothed, we have the "virgate" type (C), which is frequently {211}asymmetrical. In the "submalleate" type (B) the mallei only are slender; in the "forcipate" type (D) both the unci and rami are slender and sharply pointed.[257] In the "malleoramate" type (E) the manubrium is a curious looped structure, while the uncus is formed of a number of parallel slender elongated teeth; this characterises the family Melicertidae, and the genera _Triarthra_, _Pterodina_, and _Pedalion_. In the "uncinate" type (G) the mallei are simply incurved hooks with a few teeth at the free end, the rami are simple or absent, and there is no fulcrum; this type occurs in Flosculariaceae only. In Asplanchnidae the rami are large and hooked, constituting the "incudate" mastax (F); but here reduced mallei are often present, and in _Asplanchnopus_ they are almost as well developed as in Melicertidae, affording a transition to the malleoramate type. In this group too the mastax has a very peculiar form; it is divided into two chambers, dorsal and ventral. The dorsal chamber forms a great purse-like sac or crop, with a framework of four longitudinal bars: into this the gullet and pharynx open. The ventral pouch is much smaller, and in its base the large rami are inserted, so that they can be protruded into the crop. This ventral sac with the rami may even be everted through the crop and the mouth, to swallow the small Rotifers and Entomostraca which form the food of this group, or to eject the undigested remains of the food. Two lateral sacs open at the junction of the ventral pouch and the crop, but whether they play a part in the deglutition of food or in the disgorging of faeces is uncertain. The fact that the whole of this apparatus is lined by a non-ciliated chitinous cuticle justifies our view that it is simply an enlargement and specialisation of the mastax.

The trophi in Bdelloids also are only represented by the rami, which have the form of segments of a sphere, excavated on the curved sides for the attachment of muscles, and transversely ridged on the two flat sides; the gizzard is here called "ramate" (H).

It will be seen that the characters of the gizzard are very useful for classification, only breaking down indeed in the {212}Ploima; for though the majority of these present one or other of the four varieties of the malleate type, _Triarthra_ and _Pterodina_ (but not the other genera of their respective families) have the gizzard malleoramate.

The _oesophagus_ is, when present, a contractile ciliated tube in which the food makes no sojourn on its way to the stomach.

The _stomach_ may be nearly spherical, ovoid, or elongated and cylindrical. Its walls are formed of large cells, often granular and sometimes brownish, whence a hepatic function has been assigned to them. Its apertures are both surrounded by constricting muscular fibres. The _intestine_ may be simple or divided by a similar constriction into _intestine_ proper and _rectum_. The whole of the alimentary tract, with the exception of the mastax, is richly ciliated within. The rectum opens into the slender non-ciliated cloaca. The intestine is sharply bent upwards and towards the back in the tubicolous forms, but is nearly straight elsewhere; in _Trochosphaera_ and _Apsilus_ it is bent ventrally. In Asplanchnaceae and in _Paraseison_ there is no rectum, the stomach being a blind sac.