Chapter XIV: Introduction: Nematoda—anatomy—embryology—classification—ascaridae (4)

The so-called _salivary glands_, usually two in number, open into the pharynx or mastax; and the paired _gastric glands_ (Fig. 106, _gg_) open into the oesophagus or stomach. While the prehension of food is usually accomplished by the ciliary current of the disc and pharynx, we have seen that a more active swallowing action takes place in Flosculariaceae and Asplanchnidae, which devour whole Algae, Infusoria, and even other Rotifers, the long spines of _Triarthra_ not availing as a protection. Many Ploima put out the tips of their trophi to nibble at débris, or, in the case of _Diglena_ and _Distemma_, to attack Desmids, or the Infusorian _Stentor_. But this use of the trophi is most efficient in _Ploesoma_. Bilfinger[258] writes: "It has the courage to attack larger Rotifers; thus I was able to observe under the microscope how it fell upon a _Rattulus_ but little smaller than itself and destroyed it. First it plunged the sharp prongs of its mastax deep into the tender frontal area of its unhappy victim; then followed a pumping action of the gizzard, and stroke by stroke the whole contents of the victim's body passed into the brigand's stomach." From this it is an easy transition to the ectoparasitism of _Drilophagus_, _Balatro_, and {213}some species of _Albertia_, which cling to their host by the exserted trophi.

RENAL ORGANS.—The kidneys consist of a pair of convoluted tubes, formed of a succession of perforated, so-called "drainpipe" cells (Fig. 106, _k_); they open directly or indirectly into the cloaca. Their walls are thin in the straight parts, but thick and glandular in the coils which occur at intervals. These tubes bear little tag-like appendages, hanging freely into the body-cavity, often widening towards the free end, and flattened or circular in section (Fig. 106, _ns_). They show during life a peculiar flickering motion in their interior, like the equivalent "flame-cells" of many Platyhelminthes (see p. 25), and are in function the representatives of the multicellular renal funnels of Annelids. On one side, especially on the edge of the flattened tags, the appearance is as of a tapering whip-like lash, attached by its base to the free end of the tag and waving in its cavity; but the side view of the flattened tags shows an appearance of successive transverse or oblique waves. In many if not all cases the free end of the tag is closed by a vacuolated plug of protoplasm, which sometimes at least bears two flagella waving freely in the body-cavity. The probable explanation of the two distinct wave appearances within the tag is that the protoplasmic plug bears on its inner face a row or tuft of long cilia hanging down into the cavity of the tag. The tags probably keep up a current of liquid through the kidneys, while the contents of the body-cavity are constantly replenished by osmosis.

The two renal tubes may end blindly below the disc, or else join by a short transverse dorsal communication in front of the brain, as in _Stephanoceros_, _Atrochus_ (Fig. 112, C), and _Apsilus_ among Flosculariaceae, _Lacinularia_ among Melicertidae, and _Hydatina_ among the Illoricate Ploima (Fig. 106, _rc_). In some species of _Asplanchna_, if not all, a recurrent branch occurs opening at either end into the main tube of its own side.

The kidneys unite to discharge into the cloaca near its orifice, and on its distal (primitively ventral) side in many Melicertidae. In Bdelloida the common duct formed by their fusion opens into the ventral side of a dilated bladder-like section of the cloaca (Fig. 109, A, _bl_), which contracts rhythmically to discharge the liquid; while in the majority of the class they open singly or by a common duct into a separate contractile vesicle or _bladder_, which also discharges at regular intervals into the cloaca on its ventral or distal side (Figs. 106, _bl_ and 112).

{214}

This bladder may reach when expanded one-third the diameter of the whole animal, and contract as often as three times per minute; so that in a period of nine minutes a bulk of water equal to that of the animal must have diffused through the body-wall, to be removed by the kidneys. It is obvious that while the function of the kidneys is primitively excretory, the passage of the water through the body must bring in the oxygen dissolved in the external {215}medium, and carry off the carbonic acid formed in the tissues, and so fulfil the act of respiration. This mechanism is physiologically comparable with that of the contractile vacuole of fresh-water Protozoa. In a few genera (_Conochilus_, _Lacinularia_, _Pterodina_) the kidneys open separately after a slight dilatation into the cloaca.

NERVOUS SYSTEM.—The nervous centre of the Rotifera is the _brain_ (Fig. 112, C, _br_), a ganglion lying dorsal to the pharynx; and when this is short it may be immediately below the surface of the disc (_Microcodon_). In Bdelloida a second ganglion is present below the pharynx, and is connected with the former by lateral cords which contain ganglion cells. From the brain, nerves are given off to the disc, to the muscles, and to the integument of the body, as well as to the sense organs. The largest nerves are two given off from the sides of the brain, each of which divides into a lateral and a ventral trunk, which run nearly the whole length of the animal.

The brain of several Notommatidae has a curious appendage, white by reflected light and very opaque; it is a sac full of chalky mineral matter, which dissolves readily in dilute acids.

SENSE ORGANS.—The most widely diffused sense organs are the antennae or feelers, which may serve for touch or smell, or possibly both. Each antenna is a conical or tubular outgrowth of the skin; from its apex projects a fine pencil of sense hairs borne on a protoplasmic cushion, which receives a nerve. Often the antenna is elongated, and may then contain a muscle by which it is retractile (lateral antennae of _Melicerta_); sometimes it is reduced to a slight prominence bearing the setae (dorsal antenna of this genus). There are usually three antennae—a _median dorsal_ (Figs. 109, B, _a_, and 112, C, _am_) and two _lateral_ (Figs. 106, 112, C, and 115, A, _al_), often approximated towards the ventral surface, and sometimes all but fused on the middle line, or completely united (_Conochilus dossuarius_, _Copeus caudatus_).[259]

Most Rotifers possess an organ of sight. This in its simplest form is a refractive globule seated in a red pigmented cup through which the nerve passes; in other cases it lies directly on the brain. Very frequently the eye is paired (Figs. 112, B, and 115, A); and these paired eyes may lie on the brain, and then {216}are so close together that the pigment-cups have the shape of an _x_, or else they are seated in the dorsal region of the head behind the disc. In some cases they lie just under the ciliary wreath, or even within the region of the disc, and pass towards its ventral side in _Pedalion_ (Fig. 117, A, _e_). In _Rotifer_ they lie just under the dorsal side of the proboscis just below its apex. The median and two lateral eyes often exist together, as in _Eosphora_; and sometimes additional paired eyes exist. In _Furcularia longiseta_, var. _grandis_ a pair of pigment spots (eyes?) occurs at the hinder end of the body just in front of the foot.

The active Ploima show a spontaneity of movement and marked power of avoiding obstacles, etc. This is still more marked in the very active _Pedalion_, which, as Rousselet notes, clearly avoids capture by the dropping tube, aided by its sense of sight, as he suggests, or by the tactile or olfactory powers of the antennae. They must rank as psychically high in the scale of creatures of simple organisation.

REPRODUCTIVE ORGANS AND REPRODUCTION.—The most conspicuous organ in the female is the large _yolk-gland_ or _vitellarium_ (Figs. 106 and 109, A, _vm_), which was regarded as the ovary by all the older observers. It consists usually of eight cells, with conspicuous nuclei, lying on the ventral side of the stomach, and frequently displaced to one side; but in most Asplanchnidae it forms a broad transverse band of numerous cells. In _Pterodina_ it is horseshoe-shaped, while in Seisonaceae and Bdelloida it is paired, either gland containing four or eight cells. The true _ovary_ or _germarium_ (Fig. 106, _gm_) lies more or less hidden between the yolk-gland and the stomach; it is composed of numerous minute rounded cells, of which the hindmost for the time being enlarges by nutrition from the yolk-gland, and finally receives a membranous shell. This true ovary is somewhat lateral in most Rotifers, but is median in Asplanchnidae, and paired in _Pterodina_, Bdelloida, and Seisonaceae. A membranous covering is common to the ovary and yolk-gland (paired when these are paired); it is continued into a thin-walled tube or oviduct, which opens into the cloaca on its ventral side beyond the bladder or common renal duct. In the viviparous species the mature ovum (Fig. 112, _em_) usually lies in the oviduct, dilating it into a sort of "uterus" until the birth of the young. The ordinary eggs or "summer eggs" are formed without any {217}fertilisation, and develop immediately; they are often hatched within the tube of the tubicolous species.

Under certain conditions the unfertilised females produce exclusively smaller eggs, which develop into males. Maupas[260] has demonstrated that a rise in temperature to a minimum of 26° C. (79° F.) is the efficient factor. But as Bergendal points out,[261] the critical temperature probably varies with the antecedent conditions of the race, since males occur in Greenland at a very much lower temperature; and it would seem probable that a temperature approaching that at which the pools habitually dry up is what is necessary for the production of males, as a provision for those fertilised eggs, which, having a hard shell often adorned with prickly prominences, and usually remaining for some time before development, are capable of withstanding drought; such eggs are termed "winter eggs," but a better term would be "resting eggs" (German, "Dauereier").[262]

The male organs consist of a testis (Fig. 113, A, _te_) with accessory glands, a large seminal vesicle, and a protrusible or projecting penis (_p_). In _Notommata_ and _Diglena_ true intromission at the cloaca (B) has been seen by many observers; but it {218}appears equally certain that in many cases the male bores into the body-wall of the female at any point, and deposits the spermatozoa in the body-cavity, so that they must pass through the wall of the oviduct to effect fertilisation. Maupas finds that the process of fertilisation is ineffective except upon such newly-hatched females as would otherwise be the parents of small male eggs; that fertilisation is inoperative even for these at a later age when their eggs have begun to mature; and that it is wholly useless for those that lay ordinary summer eggs. The parent of male or winter eggs would thus be comparable to the queen bee, which if not fertilised produces drones. These sexual relations find a close parallel in the Ostracod and Phyllopod Crustacea, as well as in many plant-lice (Homoptera).

DEVELOPMENT.—This has only been fully studied in the summer egg; in _Brachionus_ by Salensky,[263] in _Melicerta_ by Joliet[264]; in _Eosphora digitata_ and several other species by Tessin[265]; in _Callidina_ and _Melicerta_ by Zelinka,[266] the last two observers having utilised modern methods of research.[267] We shall base our account on Zelinka's observations. As in the case of most "parthenogenetic" eggs, the ovarian egg begins by a very uneven division to form two cells: the minute "first polar body" which undergoes no further development; and the definitive egg, which by its repeated divisions gives rise to the tissues and organs.

Segmentation is very unequal, and recalls that of Molluscs in several respects. The first division gives rise to a smaller and a larger cell. Both of these divide again, the latter unequally, so that now there are three smaller cells and one large one; and after repeated divisions of the small cells and unequal divisions of the larger one, a stage is reached where there are a number of small cells and one large one, which sinks in and is overgrown by the small ones. Just prior to this the large cell undergoes equal divisions; its cells are the "hypoblast" cells (Fig. 114, _hyp_), and give rise to the gullet, stomach, and intestine, with their appendages, and the generative organs; while the smaller cells constitute the "epiblast" (_ep_), which gives rise to the body-wall and muscles, to the cement glands, nervous system, pharynx and mastax, and probably to the kidneys.

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Owing to the elongation of the body within the narrow space of the egg the hinder part is bent up on the ventral surface (D, E); and this part, narrower than the rest, forms the foot, the centre of which is at first occupied by a column of hypoblast. The cloaca is now formed by a dorsal ingrowth of epiblast (the "proctodaeum") at the junction of the foot and the body (_an_). The hypoblast in the body anterior to the cloacal ingrowth forms the digestive apparatus; the part immediately behind forms the reproductive organs (_o_); and the hindmost part apparently disappears. An ingrowth of epiblast at the extreme tip of the foot gives rise to the cement glands (_fg_). The muscles arise from the epiblast cells. The disc arises from the modification of epiblast cells lateral to and behind the mouth, enclosing a so-called "polar area"; it is completed by the transformation of cells on the ventral side of the mouth. The brain (_br_) is formed by the multiplication of epiblast cells; and in Bdelloida a ventral ingrowth below the mouth forms the sub-oesophageal ganglion. The ciliated cup in _Melicerta_ is formed as a ventral hollow, only later on united with the ciliated furrow of the wreath by the lateral grooves.[268] In _Melicerta_ the two eyes are formed in the polar area. The young as hatched {220}differs from the adult in the greater simplicity of its ciliary wreath; and in the tubicolous forms the cupped end of the foot-gland is ciliated, and two eyes are present on the polar area, which later sink in, and often disappear more or less completely. It is stated that the young hatched from winter eggs do not pass through this larval state.

CLASSIFICATION.[269]

ORDER I. FLOSCULARIACEAE.—Females mostly tubicolous, attached by a long contractile foot. Disc produced into a wide funnel-shaped contractile cup, produced into lobes with long setae (_Floscularia_) or coarse cilia (_Stephanoceros_), or entire (Apsilidae); an outer row of fine cilia rarely present; trochus a horseshoe, open behind. Oral funnel a slender tube hanging freely into a large pharyngeal crop; trophi uncinate projecting freely into the crop. Kidneys often united by an anterior cross-piece. Body-wall often containing a definite system of canals, filled with refractive granules, and serving by their contraction to {221}dilate the disc. Males (Fig. 107, 1) and larvae vermiform with a ciliated pedal cup, and a simple wreath, with two eyes on the disc.

Fam. 1. Flosculariidae: _Floscularia_ Oken, _Stephanoceros_ E. (Fig.
115).

Fam. 2. Apsilidae: _Apsilus_ Metschnikoff (Fig. 112, A), _Acyclus_ Leidy,
_Atrochus_ Wierzejski (Fig. 112, C).

The family Flosculariidae contains some most exquisite forms; _Stephanoceros_, the "Crown Animalcule," being probably the most lovely of the Class, and many of the Floscules coming not far behind. The Apsilidae are mostly mud-dwellers.

ORDER II. MELICERTACEAE.—Females (except in _Trochosphaera_) attached or tubicolous; tube variable. Disc with a dorsal gap (except _Conochilus_) often two-lobed or corolla-like; a ventral lip often separating off a ventral ciliated cup continuous by a pair of gutters with the ciliated groove; trochus of stronger cilia than the cingulum. Trophi malleoramate in a distinct mastax. Intestine much curved dorsally, cloaca long eversible (except _Trochosphaera_). Males and larvae as in Order I.

Fam. 3. Melicertidae: _Melicerta_ E. (Fig. 116), _Limnias_ Schrank,
_Cephalosiphon_ E., _Oecistes_ E., _Lacinularia_ E., _Megalotrocha_ E.,
_Conochilus_ E., _Octotrocha_ Thorpe.

Fam. 4. Trochosphaeridae: _Trochosphaera_ Semper (Fig. 118, D).

The Melicertidae embrace a large number of tubicolous forms, many of which are social. This habit is especially noticeable in _Lacinularia socialis_, which forms a gelatinous incrustation easily seen by the naked eye; and in _Conochilus volvox_, which forms free-swimming globular aggregates, the young attaching themselves when hatched to the centre of the ball, and the ball splitting up into two as soon as undue pressure is exerted at the periphery by overcrowding. In this genus the eyes are very conspicuous in the adult, as they are in the similar free-swimming aggregates of _Lacinularia racemovata_.

_Trochosphaera_ (Fig. 118, D) is remarkable for its peculiar {222}spherical shape, the absence of a foot, the limitation of the viscera to the lower hemisphere, and the dorsal position of the ovary. But a reference to the figure will show that the outgrowth of a foot in the quadrant between the mouth and anus and the flattening of the upper hemisphere would bring its organs on the whole into close correspondence with those of the rest of the Order. It is recorded from South China, the Philippines, and North-East Australia, and has only been seen by Semper, the founder of the genus, and by Thorpe, who saw the male of the first species, and described a second.[270]

ORDER III. BDELLOIDA.—Females creeping like a leech, as well as swimming (males unknown), susceptible of desiccation and revival ("anabiotic"). Body telescopic at both ends. Disc (except in _Adineta_) chiefly composed of two dorsal lobes like kettle-drums, wholly retractile; a dorsal proboscis or trunk-like prolongation of the body ends in a ciliated, sensory, and adhesive cup used in crawling, and overhung by a pair of membranous flaps. Trophi ramate; brain with a ventral ganglion, forming a complete ring. Eyes, two on the proboscis or brain, or absent. Bladder a mere dilatation of the rectum. Foot often possessing blind spurs, as well as two or three retractile perforated toes, or forming a terminal disc perforated by numerous pores of the cement glands, rarely ciliated.[271]

Fam. 5. Philodinidae: _Philodina_ E. (Fig. 110), _Rotifer_ Schrank,
_Actinurus_ E., _Callidina_ E. (Fig. 109), _Adineta_ H.

This group is remarkable for the great resisting powers of its members to drought and to heat and cold when dried, a fact which may explain the absence of males, though Janson records the occurrence of winter eggs in four species of _Callidina_ and in _Adineta vaga_. The body is often strongly pigmented; red in _Philodina roseola_, _Callidina scarlatina_, and _C. russeola_, yellow in _P. citrina_, _Rotifer citrinus_, and _Discopus synaptae_. Most of the species are dust- or moss-dwellers; some, such as _Rotifer vulgaris_, are equally common in organic débris in infusions, pools, and ditches. _Discopus_ adheres to the skin of the Holothurian _Synapta_.

ORDER IV. ASPLANCHNACEAE.—Females ovoid, footless except in _Asplanchnopus_. Disc often bearing a pair of antennae; circular, often prolonged at the margin into two rounded lobes, interrupted {223}dorsally, depressed at the ventral side into a deep ventral funnel. Trophi incudate (virgate in _Ascomorpha_), mastax enlarged dorsally into a wide crop; stomach large, blind. Kidneys large, with a "recurrent duct" and numerous tags; bladder large. Brain large, with a median eye, and frequently paired smaller eyes at the base of the marginal processes of the disc; anterior antennae paired, relatively far back on dorsal surface. Males (Fig. 107, 5) relatively large, frequently found.

Fam. 6. Asplanchnidae: _Asplanchna_ G., _Asplanchnopus_ De Guerne, (?)
_Ascomorpha_, Perty, (?) _Dinops_ Western.

ORDER V. SCIRTOPODA.—Females of conical shape, with the body prolonged into hollow limb-like expansions (see p. 201) moved by strong muscles, and ending in branched setose fins like the limbs of Crustacea. Disc as in Bdelloids, but not retractile. Foot represented by two subventral toes, ciliated, inconstant or absent. Trophi malleoramate. Eyes two, latero-ventral, on the disc. Male (Fig. 107, 8) conical, with simple setae.

Fam. 7. Pedalionidae: _Pedalion_ H. (Fig. 117),[272] _Hexarthra_
Schmarda.[273]

ORDER VI. PLOIMA.—Free-swimming forms, more rarely parasites, often adherent by their trophi to a host. Disc variable, often bearing within the cingulum a number of lobes fringed with coarse compound cilia. Foot rarely absent, marked off by a sharp constriction. Mastax variable, rarely malleoramate, never incudate or uncinate. Intestine not blind. Males small.[274]

SUB-ORDER A. ILLORICATA.—Ploima with a soft flexible integument; disc variable; ciliated auricles sometimes present (Synchaetidae, Notommatidae); foot rarely absent; trophi usually malleate.

{224}Fam. 8. Microcodonidae: _Microcodon_ E., _Microcodides_ Bergendal.

Fam. 9. Rhinopidae: _Rhinops_ H.

Fam. 10. Hydatinidae: _Hydatina_ E. (Fig. 106), _Notops_ H., _Hudsonella_
Zach., _Cyrtonia_ Rouss.

Fam. 11. Synchaetidae: _Synchaeta_ E.

Fam. 12. Notommatidae: _Notommata_ E., _Pleurotrocha_ E., _Copeus_ G.,
_Proales_ G., _Furcularia_ G., _Eosphora_ G., _Triophthalmus_ E.,
_Diglena_ E. (Fig. 113), _Distemma_ E., _Triphylus_ E., _Taphrocampa_ G.,
_Albertia_ Duj., _Balatro_ Clap.

Fam. 13. Drilophagidae: _Drilophagus_ Vejdovsky.

Fam. 14. Triarthridae: _Triarthra_ E., _Polyarthra_ E., _Pteroessa_ G.,
_Pedetes_ G.

To this group belongs the eyeless _Hydatina_, a classical object of study, common in greenish pools, whose male was the first male Rotifer to be figured by Ehrenberg (1838), though he did not recognise its nature, and gave it the name of _Enteroploea hydatina_. _Rhinops_ has the back of the corona curiously prolonged forwards into a sort of proboscis bearing two eyes. Some species of _Notommata_ and _Proales_ are distinctly annulated; in _Taphrocampa_ the segmentation is so marked as to give the appearance of mesenteric septa extending inwards from the body-wall to the intestine. _Microcodon_ has a wreath which is very peculiar in its extreme simplicity, with the mouth nearly central, and the eye lying just dorsal to the mouth. The Triarthridae, which resemble the Scirtopoda in having strong leaping spines fringed by fine bristles, should perhaps be placed in the next sub-Order.

SUB-ORDER B. LORICATA.—Ploima with a firm elastic cuticle {225}of definite form, persistent after death, continuous, or divided by thinner strips into plates or shields, which again may be areolated. The cuticle may also be shagreened or embossed in various ways.

Fam. 15. Rattulidae: _Rattulus_ E., _Mastigocerca_ E., _Coelopus_ G.,
_Diurella_ (?) Eyfurth.

Fam. 16. Dinocharididae: _Dinocharis_ E., _Scaridium_ E., _Stephanops_ E.

Fam. 17. Salpinidae: _Salpina_ E., _Diaschiza_ G., _Ploesoma_ Herrick,
_Diplax_ G., _Diplois_ G.

Fam. 18. Euchlanididae: _Euchlanis_ E., _Dapidia_ G., _Apodoides_ Joseph.

Fam. 19. Cathypnidae: _Cathypna_ G., _Distyla_ Eckstein, _Monostyla_ E.

Fam. 20. Coluridae: _Colurus_ E., _Metopidia_ E., _Monura_ E., _Mytilia_
G., _Cochleare_ G., _Dispinthera_ G.

Fam. 21. Pterodinidae: _Pterodina_ E., _Pompholyx_ G.

Fam. 22. Brachionidae: _Brachionus_ E., _Noteus_ E., _Schizocerca_ Daday.

Fam. 23. Anuraeidae: _Anuraea_ E., _Notholca_ G., _Eretmia_ G.

The group includes a number of very minute forms, besides others conspicuous both for size and beauty. A soft dorsal flap above the head occurs in _Stephanops_; also in Coluridae, a large family of minute species, where the flap is movable, and looks in profile like a hook overhanging the forehead. The genus _Pterodina_, like _Pedalion_ and _Triarthra_, combines a Bdelloid disc with malleoramate trophi, while its exsertile wrinkled foot ends in a ciliated cup like that of a larval tubicolous species.

_Brachionus_, a large, often flat, transparent form, with a long wrinkled foot, is a very common genus, known to the earlier observers, and repeatedly figured by them. _Pompholyx_ has a sack-like lorica, no foot, and carries its immense egg suspended by an elastic thread from the cloaca. The Anuraeidae lack the foot, and often have great spines or bristles projecting from the lorica, which no doubt facilitate floating. They are abundant in the "plankton" or floating fauna of large lakes far from the shore. Many marine species belong to this family.

ORDER VII. SEISONACEAE.—Marine Rotifers parasitic on the Crustacean _Nebalia_; males resembling the females. Body elongated, with a slender retractile neck, a much reduced disc, an elongated foot with a terminal perforated disc as in _Callidina_. Trophi virgate exsertile. Genito-urinary cloaca opening at the base of the neck in the male, at the hinder end of the body in the female. Intestine complete (_Seison_) or blind (_Paraseison_).[275]

{226}Fam. 24. Seisonidae: _Seison_ Grube; _Paraseison_ Plate;
_Saccobdella_ Van Beneden and Hesse.

HABITS.—The habitat of Rotifers is well known to the student of pond life. Every dip from a greenish pool will give us a supply, if there be not an excessive contamination by manure; and such pools give us some of the largest and most beautiful forms, such as _Hydatina_ and _Brachionus_, swimming about among the fibrous Algae and feeding on the organic débris among them. Almost any organic infusions freely exposed to the open air will yield Ploima shortly after the active putrefaction is completed. The finer water-weeds yield most of the beautiful tubicolous forms. A whole group of species and genera are quasi-pelagic in fresh and salt water, constituting a large proportion of the "plankton" or floating life near the surface; and some of these are found in deep water or in the depths of the lakes. Among them are the Asplanchnidae, Triarthridae, and Anuraeidae. A number of Loricates, such as _Notholca_ and _Eretmia_, are armed with long spines, which doubtless render floating easier.

Among tubicolous forms _Conochilus volvox_ and _Lacinularia racemovata_ have this pelagic habit, forming floating globular or ovoid colonies, and two species of _Floscularia_ also float freely in their tubes.

The following forms occur in salt or brackish water,[276] those marked with an asterisk (*) also occurring in fresh water:—

_Floscularia campanulata.* Melicerta tubicolaria.* Rotifer citrinus.*
Discopus synaptae. Synchaeta baltica, S. monopus, S. apus, S. tremula,*
S. longipes, S. tavina. Asplanchna girodi.* Asplanchnopus syringoides.
Hexarthra polyptera. Notommata naias, N. reinhardti. Proales decipiens.
Furcularia forficula,* F. gracilis, F. reinhardti, F. marina, F.
neapolitana. Diglena catellina,* D. suilla, D. putrida. Pleurotrocha
leptura. Distemma raptor, D. marinum, D. platyceps.* Bothriocerca
longicauda. Polyarthra platyptera.* Triarthra longiseta.* Rattulus
calyptus. Diurella marina, D. brevidactylus, D. brevis. Diaschiza
fretalis. Euchlanis luna. Monostyla quadridentata, M. lunaris. Colurus
amblytelus, C. uncinatus,* C. dactylotus, C. coelopinus, C. pedatus, C.
rotundatus, C. truncatus, C. caudatus.* Mytilia tavina. Pterodina
clypeata. Brachionus bakeri,* B. mülleri. Anuraea valga,* A. biremis,* A.
aculeata,* A. tecta,* A. cochlearis.* Notholca striata,* N. scapha,* N.
thalassia, N. spinifera, N. inermis, N. jugosa, N. rhomboidea. Seison
grubei, S. annulatus. Paraseison asplanchnus, P. nudus, P. proboscideus,
P. ciliatus. Discobdella nebaliae._

Thus about seventy species are recorded as marine. _Synchaeta baltica_ is truly pelagic, and contributes to the phosphorescence of the ocean.

{227}Other forms again are parasitic. _Proales werneckii_ is found in _Vaucheria_, a coarse, dark green, thread-like Alga found in fresh water; and the closely allied _P. parasita_ is not uncommon in the beautiful floating green spheres of _Volvox_.[277] _Albertia_, _Drilophagus_, and _Balatro_ are parasitic on or in fresh-water Oligochaetes; the curious Seisonaceae are parasitic on _Nebalia_, a small Crustacean easily obtained in masses of whelk's eggs; the aberrant Bdelloid _Discopus_ attaches itself to the surface of the Holothurian _Synapta_. Similarly among this last Order _Callidina parasitica_ attaches itself to the limbs of the fresh-water Crustacea _Gammarus_ and _Asellus_. These are rather commensals than true parasites. The species of _Brachionus_ often attach themselves temporarily to the common water-flea _Daphnia_.

Besides a few Ploima, the vast majority of the Bdelloids live in or among mosses and their roots. Many _Callidina_ inhabit cup-like hollows in the leaves of the scale mosses (Jungermanniaceae), especially of the genus _Frullania_. Almost all the members of this Order are susceptible of desiccation and revival; certain species, such as _Rotifer vulgaris_, _Philodina roseola_, _Adineta vaga_, etc., can be readily obtained by moistening gutter dust. The mechanism of the process is as follows: when desiccation is gradual the animals close up their telescopic bodies and excrete gelatinous plugs at either end, which effectually seal them against further drying; if, however, they be dried on a slide without any débris, the process is too rapid for them to protect themselves, and they therefore die. This was dimly seen by others, and clearly demonstrated by H. Davis,[278] who records the following experiment:—The Rev. E. J. Holloway, having found _Philodina roseola_ in gutters, placed strips of paper there in the rainy season, and succeeded in obtaining clean gatherings, taking dry groups of a hundred together, having a varnish-like covering all over; and being glued to one another, mostly in one plane, and to the paper, forming a pavement. In the dry condition they resist extremes of temperature; thus Zelinka found {228}_Callidina_ revive after an exposure of -20° C. (-4° F. or 36° of frost), and immersion in hot water at 70° C. (158° F.). They will also resist deprivation of air in a vacuum of an ordinary air-pump, but not the all but perfect exhaustion of the Sprengel pump.

A very curious fact in relation to this Class is that often when a new form is once described from a single locality, fresh and widely distant stations for it rapidly become known.[279] Thus _Pedalion mirum_, first found at Clifton in 1872 by Hudson, was a few years after captured in a small pool above tide-marks on a rocky islet in Torres Straits. Since then it has been recorded from many different European stations, and a second closely allied species has been found in Finland. So a species of Ehrenberg's[280] was not seen again till within the last decade or so; but since then it has been independently found and described by six observers, who have given it as many distinct generic names. In the case of _Pedalion_ it may well be that, as Hudson suggests, the species is of southern origin and has followed the flag, the winter egg being conveyed in dust by ships or travellers.

The above account of the habits gives the key to the collection of the various forms. The weed-loving species are collected with the weeds, and will keep with these in vessels if screened from direct sunlight and protected against dust. The free-swimming forms may be collected by sweeping with a net of fine gauze, with a bottle fixed in the bottom.

Except for their power of resisting desiccation, Rotifera are not very long-lived, and the males are especially short-lived; the most exact observations are those of Maupas on _Hydatina_. He found that the greatest age of the unfertilised female was thirteen days, during which it could produce some fifty eggs; the fertilised female lives for seven or eight days, producing about sixteen eggs; while the male dies in two or three days.

The preservation of Rotifers has been recently reduced to a fine art by Rousselet, who uses a solution consisting of cocaine hydrochlorate, 1 gramme; water, 50 cc.; and methylated spirit, 12 cc. This will keep without deterioration. When in use it {229}must be diluted in the proportion of two volumes to three of water. This solution is added cautiously to the capsule in which the Rotifers lie, and they are watched till their ciliary motions slacken; when this happens a drop or two of osmic acid solution (½ to 1 per cent) is added; the Rotifers are then sucked up by a capillary pipette, and transferred to fresh water; and then into a solution of "Formaline" diluted to contain 2½ per cent of formic aldehyde. In this solution they are transferred to shallow cells, ground out of the centre of an ordinary glass slide, covered with thin glass, and sealed.[281] Other methods of preparing Rotifers for minute study will be found in the papers of Plate, Tessin, and Zelinka.

THE ZOOLOGICAL AFFINITIES of the Rotifers have long been a subject of keen interest. As early as 1851 Huxley[282] suggested that they represent a primitive form, preserved, with modifications, in the larva of Molluscs, Annelids and other worms, and Echinoderms. Similar views were later maintained by Lankester,[283] who termed the larva of Polychaets, etc., a "trochosphere," for which "trochophore" has been substituted in order to avoid confusion with the Rotifer _Trochosphaera_; Balfour,[284] Hatschek,[285] Kleinenberg,[286] and others have developed these views. Serious difficulties, however, arise in the detailed comparison of Rotifers with this type; and the special students of this Class have found it practically impossible to agree in the identification of the various parts, a difficulty especially felt in the case of the Rotiferan genus _Trochosphaera_, though this is just the one which presents the closest superficial resemblance to the Trochophore larva. I have been induced to take a view of the structure of Rotifers that brings it into close relationship with the lower Platyhelminthes, and with the more primitive larva of the Nemertines termed _Pilidium_ (Fig. 60, p. 113). This is hemispherical, ciliated all over, with the mouth in a ventral funnel lined by fine cilia; while the edge is fringed with two rows of strong cilia, separated by a finely ciliated groove, like those of the ciliary wreath of a Rotifer.

{230}

The sides are produced on either side into lappets, which we do not take into account. A cup-shaped depression at the apical pole is lined by sense-cells, bearing long cilia which are probably sensory. A ring of nerve-cells passes within the ciliated rim of the hemisphere, and the stomach is a blind sac. If we compare this organism with a Rotifer, we find that the wreath corresponds in both, the funnel of the disc in such forms as Flosculariidae and _Microcodon_ leading to the mouth of _Pilidium_, while the gut is blind in Asplanchnidae and in some of the highly developed Seisonidae. The circular nerve-ring of _Pilidium_ is in many Rotifers only represented by its anterior part, the brain; though in Bdelloids a sub-oesophageal ganglion completes the ring. This leaves a difficulty with regard to the apical sense organ; but it is easy to understand that an organ of sensation should become an organ of fixation. In this case the foot with its glands would correspond to the sense organ of the Trochophore larva; and it retains its primitive ciliated character in the larvae and males of many Rotifera, and the adult female of _Pterodina_ and _Callidina tetraodon_. Embryology tells {231}us that the anus of Rotifers cannot be homologous with that of Annelids, etc., for it is formed outside the area of the blastopore: it is an independent formation, probably due to the coalescence of the originally blind intestine at its extremity with the earlier genito-urinary cloaca. On this view we must change the orientation of the Rotifer, and place it, like a Cuttlefish, mouth downwards: for "anterior and posterior" we must substitute _oral_ (or _basal_) and _apical_; for "dorsal" and "ventral" we must use _anterior_ and _posterior_; while "right" and "left" are unchanged. And this correctly expresses the actual space-relations in those Ploima like _Rattulus_ that swim with their disc in contact with the organic débris on which they feed, with the foot turned outwards and backwards. As these views are now published for the first time, I have thought it wiser to keep to the accepted relations in the general description, a course which has the advantage of avoiding difficulties in the study of the literature of the Class.

The supposed resemblance of _Pedalion_ to the Crustacea is probably the result of convergence, not of consanguinity. The Polyzoa are a group of freely-budding organisms whose structure otherwise recalls in many respects that of the attached Rotifers; but a close investigation reveals so many differences in structure, orientation, and development, that we cannot regard the two groups as at all closely allied.

Thus the Rotifers may be regarded as a group apart, but probably representing an early offshoot from a free-swimming Platyhelminth, probably a Rhabdocoele; the modifications being the loss of the general ciliation of the surface, the arching of the back into an elongated vault, the conversion of the inner half of the pharynx into a gizzard, the change of position of the genital and urinary apertures to the antero-dorsal surface, and the opening of the intestine into the genito-urinary cloaca.

GASTROTRICHA.

This small and very homogeneous group consists of minute fresh-water organisms, closely resembling many Ciliate Infusoria in their movements, habit and habitat. They were first described in detail by Ehrenberg, and placed by him and Dujardin in the neighbourhood of Rotifers. In recent years A. C. Stokes[287] {232}in America and C. Zelinka[288] in Germany have contributed, the former a careful description of a number of new species and their habits, the latter a complete monograph of everything that is known of the Order.

The Gastrotricha dwell among filamentous Algae and organic débris, and are of frequent occurrence with Protozoa and Rotifera of similar habit. The largest known measures only 400 µ (1/60 in.) in length, and the smallest run as low as 74 µ (1/300 in.).

We shall follow Zelinka in his description of the common species _Chaetonotus larus_ as a type. The body is nearly circular in section, flattened a little on the ventral side. The apertures are the terminal mouth; the anus, nearly terminal and slightly dorsal; the two kidney openings, ventral, nearly half-way down the trunk; besides the pore of a cement-gland on either terminal process. The short ventral and post-anal portion of the trunk with its processes therefore corresponds to the foot of a Rotifer. The integument of the body is a thin nucleated hypoderm, not {233}distinctly divided into cells, covered by a chitinised cuticle; it bears cilia, sensory hairs, and peculiar scale-like processes, sometimes produced into long bristles.

The cilia are chiefly arranged in two ventral bands, each extending nearly the whole length of the body, and composed of a series of transverse rows of single cilia; along these bands the hypoderm is thickened and more richly nucleated. The sides of the head also bear numerous long cilia.

The scales are hollow processes of the cuticle overlapping from before backwards. A ventral row lies between the ciliary bands; two series of alternating dorsal rows lie on the back and sides of the animal, and in the hirsute species it is these that are produced backward into bristles. A single large scale, the "frontal shield," protects the head above and behind, but does not extend down to the ventral surface. On either side of the head is a pair of flattened oval areas, the "lateral fields." From between these on either side springs a tuft of motile sensory hairs. Two pairs of similar tufts arise dorsally on the front margin of the frontal shield, and a fourth pair spring from the ventral surface a little behind the mouth. These hairs are distinguished from ordinary cilia by their length, and their insertion on large nucleated cells receiving nerves; two pairs of similar hairs lie farther back on the dorsal surface, one in the front of the neck, one near the base of the pedal processes.

The MUSCLES lie some in the body-wall, and some traverse the body-cavity; only six pairs occur, simple, unstriated, and longitudinal. There are neither transverse nor circular muscles.

The ALIMENTARY CANAL is very simple and nearly straight from mouth to anus; it may be divided into pharynx, gullet, stomach, and rectum. The mouth is circular, and looks forwards and a little downwards. From the mouth opens the pharynx, a short chitinous tube, capable of eversion by being pushed forwards by the gullet; it bears half-way down a circlet of curved hooks, which open out when it is everted; within these are tooth-like thickenings.

The oesophagus or gullet is thick and muscular, extending through the whole of the neck of the animal; its cavity, as well as the opening from the pharynx, is triradiate like a leech-bite, but can be dilated by the action of the muscular walls, inserted into a firm external cuticle; the internal wall is also cuticulised, {234}not ciliated as in Rotifers. The hinder end of the gullet is produced into a short, wide, membranous funnel projecting freely into the midgut or stomach. The latter is elongated and oval, composed of four rows of hexagonal cells, with large nuclei. This is separated by a distinct constriction or sphincter from the short pear-shaped rectum, which opens by a minute anus on the back just in front of the pedal processes.

The food is chiefly organic débris; but Gastrotricha have been seen to attack large Infusoria by nibbling, and to swallow the protoplasm as it exudes from the wound in their prey.

The NERVOUS SYSTEM is chiefly composed of the large brain, a ganglion lying like a saddle above and on the sides of the gullet, and in direct continuity with the nerve-cells of the cephalic sense-hairs. A pair of dorsal nerve-trunks extend along the whole length of the gullet. The sense-hairs described with the general integument may be organs of external taste ("smell") or of touch. Eyes have been described in several species; and though Zelinka has failed to verify this, I have myself seen a pair of minute red eyes in the back of the head of an animal (probably a _Chaetonotus_), whose hasty escape into a mass of débris prevented my determining its species.

The KIDNEYS are paired tubes lying at the sides of the front of the stomach, and sending a simple loop into the neck. Each tube is much convoluted, and ends at the one extremity in a long "flame-cell," like that of a Rotifer much drawn out, and at the other by a minute pore on the outer side of the ventral row of scales.

REPRODUCTIVE ORGANS.—Only the female is with certainty known to occur; and the eggs, though recalling in their thick ornamented shell the fertilised winter eggs of Rotifers, are probably unfertilised and parthenogenetic like the summer eggs. The ovaries are two minute patches of cells lying at the junction of the stomach and rectum. The eggs, as they mature and enlarge, press against the side and back of the stomach, where they attain a length of one-third to one-half that of the mother. The extrusion of the egg has not been observed; but it is laid in the angles of weeds, the moulted shells of Entomostraca, etc., where its development may be studied. The sculpture of the shell serves to anchor it if laid among weeds. When hatched the head, trunk, and pedal processes are of the full adult size, all subsequent growth being limited to the neck.

{235}The function of testis has been ascribed by Ludwig to a minute granular organ between the ovaries above the rectum; if this view be correct the Gastrotricha are hermaphrodite.

The movements of the Gastrotricha are very elegant, recalling those of the long-necked Ciliate Infusoria, like _Amphileptus, Lacrymaria_, etc., with the characteristic exception that they always swim forwards; the grace of their movements being due to the bending of the head and neck on the body. Those which are provided with long motile bristles like _Dasydetes_, alternate their gliding with leaps, like the springing Rotifers.

The Gastrotricha are divided into two sub-Orders—EUICHTHYDINA, with two pedal appendages, containing the genera _Ichthydium_ Ehr., _Lepidoderma_ Zel., _Chaetonotus_ Ehr., and _Chaetura_ Metsch.; and the APODINA, with no pedal appendages, comprising _Dasydetes_ G. and _Gossea_ Zel.

Their geographical distribution, like that of most microscopic fresh-water organisms, is cosmopolitan. Few observers have enumerated the members of this group; of their extra-temperate occurrence we have only the single observations of Ehrenberg, Schmarda, and Voeltzkow for Nubia, Ceylon, and Madagascar respectively.

Of the thirty-two species described, twelve are recorded by A. C. Stokes from Maine and New Jersey only, besides five others that occur also in Europe. In Europe nineteen species are recorded, one of which, _Ichthydium podura_, has also been found in Nubia and Ceylon. One species, _Chaetonotus tabulatus_ Schmarda, has been recorded by its author from Colombia (in South America). As of the nineteen European species only seven have been recorded as British, we may expect to find that careful study will well repay the student in these islands.

The AFFINITIES of this group are probably with the Turbellarians and the Nematodes; they differ from the former in the highly developed alimentary canal, and from the latter in the possession of the ciliated ventral bands and wreath. The general chitinisation of the skin, the primitive body-cavity, the character of the alimentary canal, the ventral opening of the renal canals far in front of the anus are characters shared by the Nematodes, many of which possess bristles like this group. But their affinity must be rather to some hypothetical ancestral group than to any living Nematodes, which are destitute {236}of cilia. To the Rotifers the affinity, dwelt on by Zelinka, is less close.

KINORHYNCHA.

This Class and Order comprises but one genus, _Echinoderes_ (Fig. 120), founded in 1851 by Dujardin.[289] Reinhard's monograph[290] is the generally accepted authority on this subject, and contains a full bibliography, with diagnoses of the individual species, eighteen in number.

The animals of this group are found in shallow seas with muddy bottom, below low-water mark, and feed on organic débris. They have been taken in the Black Sea, Mediterranean, British Channel, and North Sea, and off the Canary Islands (Lanzarote, Porto Pi, Palma di Mallorca). Their size varies from 0.86 mm. × 0.22 mm. in _Echinoderes spinosus_, to 0.14 mm. × 0.03 mm. in _E. kowalevskii_.[291]

The BODY is protected by a strong chitinous cuticle distinctly annulated, forming eleven rings, besides a retractile proboscis obscurely divided into two segments at the apex of which the mouth opens. The anus opens on the extreme end of the last segment, which is frequently retracted; the genital pores open right and left of the anus; and the renal pores lie on either side of the back of the ninth segment. The first ring may be undivided, or else distinctly divided into four plates, one dorsal, {237}two latero-ventral, and one ventral. In the remaining segments each ring has only three plates, one dorsal and two ventral, the two latter being sometimes more or less fused in the last or ventral segment. These plates all overlap from before backwards.

As the name _Echinoderes_ implies (Thorn-skin), the cuticle is produced into points, bristles, or spines. The last segment frequently bears a large pair of these, which have been compared, on the flimsiest grounds, with the furcal processes of Crustacea and the perforated toes of Rotifers and Gastrotricha.

The proboscis when extruded has the form of a truncated cone, obscurely divided into two segments, a ring of strong spines marking the boundary between them, and a second double ring of spines surrounding the apex. The eversion is of the type termed by Lankester pleurembolic or acrecbolic, the sides being first withdrawn, the apex first extruded.

As in so many Invertebrata, the epidermis is not separated by boundaries into distinct cells. This layer sends out processes each of which lies in a hollow in the thick cuticle, and perforates it to end in a fine bristle. Minute orange pigment-granules occur at irregular intervals in this hypoderm.

The MUSCLES of _Echinoderes_ are simple striated bands. Numerous bands lie within and attached to the body-wall, extending its whole length; paired dorsi-ventral muscles separate the intestine from the reproductive organ on either side, and a complex system effect the movements of the proboscis.

ALIMENTARY CANAL.—The pore at the tip of the proboscis leads into a short thin-walled tube, which is rarely evaginated; into the base of this tube projects the short bluntly conical apex of the large ovoid muscular pharynx (or gullet?); this is lined by an epithelial layer of nucleated protoplasm, which secretes a strong cuticle. The stomach is a wide tube, somewhat dilated in each segment between the paired dorsi-ventral muscles, and tapering behind to end in the terminal anus. Four minute glands open at the junction of the pharynx and stomach.

KIDNEYS.—These are a pair of blind pear-shaped sacs, ciliated within (the only case of ciliation in _Echinoderes_), lying in the eighth segment, and opening by the taper ends right and left on the back of the ninth segment.

NERVOUS SYSTEM.—All that has been clearly defined of this is a small brain or ganglion lying dorsally at the junction of the {238}pharynx and stomach. From two to eight eye-spots have been described by earlier writers, but Reinhard was unable to find them in the (distinct) species which he principally worked at, though he noted their existence in the solitary specimen of the original species, _E. dujardini_, which he obtained.

REPRODUCTIVE ORGANS.—The sexes are distinct. The reproductive glands form a pair of tubular sacs, opening ventrally on either side of the anus, and extending forwards beside the gut as far forwards as the fifth to the second segment in the male, but only to the fourth at furthest in the female. The ova are large nucleated cells embedded in the protoplasmic lining of the ovarian sac, and acquiring a distinct shell as they approach its opening. Three-quarters of the testis sac is occupied with granular protoplasm containing a quantity of small nuclei; the lower part alone contains mature spermatozoa. Adjoining each external opening in the male are a pair of short hollowed spines, which may perhaps serve as organs of copulation; but nothing is really known of this process or of the development of the egg. It is almost certain, from the absence of developing eggs within _Echinoderes_, that the genus is not viviparous.

From the foregoing description it is obvious that _Echinoderes_ approaches the Nematoda very closely: the two main points of difference are its ciliated kidneys and its bilaterally paired sexual organs. Possibly the study of such forms as _Desmoscolex_ (Fig. 81, p. 159) may reveal closer affinities.

----

[Zelinka (_Verh. D. Zool. Ges._, 1894 and 1898), has given a preliminary
account of a new research on this group. The principal addition is the
discovery of a ventral nerve-cord, with a ganglionic dilatation in each
segment, lying in the ectoderm of the body-wall, as indeed do the brain
and nerve-collar. He divides the genus into two Orders according as the
orifice of the retracted fore-part of the body is slit-like or circular.
The former (Homalorhagae) retract the first two segments with the
proboscis; they are mud-dwellers, sluggish, eyeless: the latter group
(Cyclorhagae) only retract the first segment with the proboscis; they
crawl among algae, and mostly have paired pigmented eye-spots, each with
a lens, imbedded in the brain.—M. H., _Jan._ 1901.]

ARCHIANNELIDA, POLYCHAETA, AND MYZOSTOMARIA

BY

W. BLAXLAND BENHAM, D.Sc. (Lond.), Hon. M.A. (Oxon.)

{241}CHAPTER IX

THE CHAETOPODOUS WORMS—THE ARCHIANNELIDA—ANATOMY OF _NEREIS_, AS TYPICAL OF THE POLYCHAETA

Those animals which possess lateral bundles of bristles (technically termed "chaetae") for use in locomotion constitute the group of "Bristle-worms," or CHAETOPODA. The body of these animals is made up of a preoral lobe or prostomium, and a number of more or less distinct segments following one another in a line, and repeating one another in their internal and external structure. The Chaetopoda embrace the following smaller groups or Orders:—I. Archiannelida, II. Polychaeta, III. Myzostomaria, IV. Oligochaeta. The Archiannelida, although without the characteristic chaetae, are yet anatomically so similar to the true Chaetopoda that they must be included in the group, just as certain fishes are classed as "Vertebrata," although they do not possess vertebrae. The old term Annelida is sometimes used to include the above-mentioned groups, together with the Gephyrea[292] and the Hirudinea or leeches.

ORDER I. ARCHIANNELIDA.

The Archiannelida are very simple worms, but simplicity may be, and very frequently is, the result of degeneration; and it is not always possible to determine whether a simple animal is primitively, _i.e._ ancestrally simple, or whether it is secondarily simplified. Hence the term _Haplodrili_ has been employed by Professor Lankester as the name of the group; a term which does not prejudge the question as to whether or not the worms are {242}"primitive." It is quite possible, and even probable, that _Dinophilus_ is ancestrally simple; whilst many features in _Polygordius_ appear to be the result of simplification. For this reason it would be well to separate _Dinophilus_ from the other two genera, on account of its much less elaborate and more generalised structure,—so generalised, in fact, that the worm is by some authorities placed amongst the Planarians; for the present, however, the group Archiannelida may be regarded as containing three genera: _Dinophilus_, _Protodrilus_, and _Polygordius_.[293]

_Dinophilus_ is represented on our coasts by at least two species: _D. gigas_ Weldon[294] and _D. taeniatus_ Harmer.[295] The latter is about one-twelfth of an inch in length, bright orange in colour, and more or less abundant, at springtime, in the rock pools around Plymouth, where it may be found amongst green algae, or on the mud at the bottom of the pools.

{243}The animal consists of a broad prostomium, with a pair of eyes; and of a body, distinctly constricted in immature specimens into five or six segments, followed by a short conical tail. There are neither chaetae nor tentacles; locomotion is chiefly effected by means of the bands of cilia which encircle the body in a regular fashion, two bands round the head, and two round each segment in _D. taeniatus_; in some species there is only a single band on each segment. The whole of the ventral surface is covered with cilia, by the aid of which the animal probably "creeps" along the weeds.

The alimentary canal is straight, and divisible into the regions shown in Fig. 121; a muscular protrusible organ, which is a ventral outgrowth of the foregut, is employed as a "sucker." The coelom is more or less obliterated (or ill developed). The excretory system in the genus is varied: in some species, as in _D. gigas_, it is stated to be constructed on the Planarian plan; in others, as in _D. taeniatus_, the organs are definite nephridia. Of these tubes there are five pairs, the last pair in the male serving as a seminal vesicle. Each nephridium is a ciliated tube, the internal end of which lies in the body-cavity and appears to be blocked by a ciliated tongue-shaped appendage. The first pair corresponds to the "larval nephridia" of Trochosphere larvae.

The nervous system, which is in contact with the epidermis, consists of a brain in the prostomium, and, on each side of the body, a ventral cord with five ganglia, connected by transverse commissures in as many segments.