PLATYHELMINTHES AND MESOZOA

F. W. GAMBLE, M.Sc. (Vict.)

Demonstrator and Assistant-Lecturer in Zoology in the Owens College, Manchester.

TURBELLARIA

INTRODUCTION: DESCRIPTION OF THE POLYCLADLEPTOPLANA TREMELLARIS—APPEARANCE—HABITS—STRUCTURE: POLYCLADIDA—CLASSIFICATION—HABITS—ANATOMY—DEVELOPMENT: TRICLADIDA—OCCURRENCE—STRUCTURE—CLASSIFICATION: RHABDOCOELIDA—OCCURRENCE—HABITS—REPRODUCTION—CLASSIFICATION.

The Platyhelminthes, or Flat Worms, form a natural assemblage of animals, the members of which, however widely they may differ in appearance, habits, or life-history, exhibit a fundamental similarity of organisation which justifies their separation from other classes of worms, and their union into a distinct phylum. Excluding the leeches (Hirudinea), and the long sea-worms (Nemertinea)—which, though formerly included, are now treated independently—the Platyhelminthes may be divided into three branches: (1) Turbellaria (including the Planarians), (2) Trematoda (including the liver-flukes), and (3) Cestoda (tape-worms). The Mesozoa will be treated as an appendix to the Platyhelminthes.

The Turbellaria were so called by Ehrenberg[1](1831) on account of the cilia or vibratile processes with which these aquatic animals are covered, causing by their incessant action, tiny currents ("turbellae," disturbances) in the surrounding water. The ciliary covering distinguishes this free-living group from the parasitic Trematodes and Cestodes, some of which possess such an investment, but only during their early freelarval stage, for the short period when they have left the parental host and are seeking another (Figs. 26, 27, 42).

Some Turbellaria (Rhabdocoelida) resemble Infusoria in their minute size, shape, and movements. Nevertheless they possess an organisation of considerable complexity. The fresh-water Planarians (Fig. 14), abounding in ponds and streams, vary from a quarter to half an inch in length, and are elongated and flattened. Their body is soft, and progresses by a characteristic, even, gliding motion like a snail. The marine Planarians or Polyclads (Fig. 8) are usually broad and leaf-like, sometimes attaining a length of six inches, and swim or creep in a most graceful way. Land Planarians occur in this country (Fig. 15), but far more abundantly in tropical and sub-tropical districts, in moist places, venturing abroad at night in pursuit of prey. They are elongated and cylindrical, in some cases measuring, when fully extended, a foot or more in length, and are often ornamented with brilliantly coloured, longitudinal bands.

Turbellaria are carnivorous, overpowering their prey by peculiar cutaneous offensive weapons, and then sucking out the contents of the victim by the "pharynx." Land Planarians feed on earthworms, molluscs, and wood-lice; fresh-water Planarians on Oligochaet worms, water-snails, and water-beetles; marine forms devour Polychaet worms and molluscs. Some Turbellaria seem to prefer freshly-killed or weakly examples of animals too large to be overpowered when fully active. Certain Rhabdocoelida are messmates of Molluscs and Echinoderms, and a few others are truly parasitic—a mode of life adopted by all Trematodes saveTemnocephala.

The Trematodes[2]may be divided into those living on the outer surface of various aquatic animals, usually fish (Ectoparasites); and those which penetrate more or less deeply into the alimentary canal or the associated organs of the host (Endoparasites). They are oval, flattened Platyhelminthes ranging from a microscopic size to a length of three feet (Nematobothrium, Fig. 22), and are provided with organs of adhesion by which they cling to the outer surface, or to the interior, of the animals they inhabit. Trematodes occur parasitically in all groups of Vertebrates, but, with the exception of the liver-flukes of the sheep (Distomum hepaticumandD. magnum), and ofBilharzia haematobiafound in man (in the blood-vessels of the urinary bladder) over the greater partof Africa, their attacks are not usually of a serious nature. Ectoparasitic Trematodes areMonogenetic; that is, their larvae grow up directly into mature forms. The Endoparasitic species, however, are usuallyDigenetic. Their larvae enter an Invertebrate and produce a new generation of different larvae, and these another. The last are immature flukes. They enter a second host, which is swallowed by the final Vertebrate host in which they become mature.

The Cestodes or Tape-worms have undergone more profound modifications both in structure and in mode of development. They are all endoparasitic, and, with one exception (Archigetes), attain maturity solely within the alimentary canal of Vertebrates. In length they range from a few millimetres to several metres, but this great size is attained from the need for the rapid production and accumulation of enormous numbers of eggs. The "head" or "scolex" is attached to the mucous membrane of the host by suckers or hooks, but there is no mouth nor any certain trace of a digestive tract at any stage of the life-history of Cestodes. For nourishment they absorb, through the skin, the previously-digested food (of the host) that bathes them. In a few Cestodes the body is simple and not divided into "proglottides" or generative segments, but in most cases it is jointed in such a way that the last segment is the oldest, and each contains a set of reproductive organs. The life-histories of Cestodes are most remarkable. The proglottides containing the eggs pass out of the final host along with the faeces and enter the intermediate host with the food. The larvae hatch, and boring their way into the blood-vessels, are carried by the circulation to various internal organs. Here they usually become "bladder-worms," and develop the "head" of the future sexual form. Then, if, as is usually the case, the intermediate host is preyed upon by the final host, the larval Cestodes enter the alimentary canal of the latter. The head of the larva alone survives digestion, and from it the mature worm is formed.

Of these three branches of the phylum Platyhelminthes, the Turbellaria possess features of special interest and importance. Not only do they furnish the explanation of the structure of the two parasitic groups (which have probably arisen from Turbellarian-like ancestors), but they occupy the lowest position in the whole group of worms. There are reasons for thinking that this is the simplest group of bilateral animals which adopt the habit ofcreeping. The Turbellaria are most closely allied to that great extinct group from which they, the Nemertinea, Rotifera, and even the Annelids, offer increasingly convincing evidence of having been derived. Many questions relating to the affinities of, or the origin of organs in, the Annelids, resolve themselves into similar questions about the Turbellaria. For these reasons, this group is here dealt with at greater length than the others, the interest of which is of a more special nature.

The history of our knowledge of the Cestodes dates back to ancient times, as the presence and effects of tape-worms early attracted the attention of physicians. Trematodes are first distinctly referred to in the sixteenth century, while Turbellaria first figure in Trembley's memoir on Hydra (1744).[3]The whole subject of the increase in our knowledge of parasitic Platyhelminthes is dealt with in the standard work,The Parasites of Man, by Leuckart,[4]and a complete list of references in zoological literature to Cestodes and Trematodes is to be found in Bronn'sThierreich.[5]O. F. Müller[6]and Ehrenberg founded our knowledge of the Turbellaria, but for a long time the group remained in a most neglected condition. In this country Montagu, G. Johnston, and in Ireland, William Thompson, discovered several marine species, one of which,Planocera folium(from Berwick), has not again been met with on British shores. Dalyell[7]conducted classical researches on the habits of Planarians, and Faraday[8]made interesting experiments on their power of regenerating lost parts. The credit of assigning the correct interpretation to most of the various organs of fresh-water Planarians belongs to von Baer[9]and Dugès,[10]while Mertens[11]effected a similar service for the marine forms, or Polyclads. The minute Rhabdocoels were first successfully investigated and classified by Oscar Schmidt.[12]The great work on this group is, however, themonograph by von Graff.[13]A similarly comprehensive and indispensable treatise by Lang, on the Polycladida,[14]contains references to all previous publications on the group, among which the papers by Quatrefages, Johannes Müller, Keferstein, Minot, and Hallez stand out conspicuously. Moseley's work[15]on the Land Planarians of Ceylon is undoubtedly the most revolutionary paper referring to this group, and the best contribution towards elucidating the structure of the Tricladida at a time when the subject was very obscure. A monograph on Land Planarians is being prepared by von Graff.

The Turbellaria are divided into: (1)Polycladida, marine forms with multiple intestinal branches; (2)Tricladida, marine, fresh-water, and terrestrial Planarians with three main intestinal branches; (3) theRhabdocoelida, as varied in habit as the Triclads, but possessing a straight and simple or slightly lobed, intestine. A detailed description of an example of the Polyclads, and then a comparative account of each division, will now be given.

fig1Fig. 1.—Leptoplana tremellarisO. F. M. Seen from the dorsal surface. The alimentary canal runs down the middle line and sends branches to the margin of the body. × 6.

Fig. 1.—Leptoplana tremellarisO. F. M. Seen from the dorsal surface. The alimentary canal runs down the middle line and sends branches to the margin of the body. × 6.

Turbellaria. I. Polycladida.

Description of Leptoplana tremellaris.

Appearance and Habits.—An account of the Polyclad Turbellaria may be fitly prefaced by a description of a very common representative,Leptoplana tremellaris, so called on account of the thin, flat body which executes when disturbed, quivering or tremulous swimming movements.

Like all Polyclads,Leptoplanais marine. It is probably found on all European shores, northwards to Greenland and southwards to the Red Sea, while vertically it ranges from the littoral zone down to fifty fathoms. There is, however, an apparently well-marked difference between the littoral specimens, which vary from three-quarters to one inch in length, are brownish in colour and firm in consistency, and the more delicate examples half an inch long, white with a brown tinge, which occur in deeper water.

fig2Fig. 2.—Leptoplana tremellaris.Three-quarters view from the ventral surface. The pharynx (ph) is widely protruded through the month (mo) as in the act of attacking prey.br, Brain with nerves, close to which are the four groups of eyes;mg, stomach;mgc, "marginal groove";pe, penis;sc, sucker;ut, uterus;vd, vasa deferentia; ♀, female genital aperture surrounded by the shell-gland; ♂, male aperture. (Semi-diagrammatic, and × 6.)

Fig. 2.—Leptoplana tremellaris.Three-quarters view from the ventral surface. The pharynx (ph) is widely protruded through the month (mo) as in the act of attacking prey.br, Brain with nerves, close to which are the four groups of eyes;mg, stomach;mgc, "marginal groove";pe, penis;sc, sucker;ut, uterus;vd, vasa deferentia; ♀, female genital aperture surrounded by the shell-gland; ♂, male aperture. (Semi-diagrammatic, and × 6.)

At low waterLeptoplanamay be found buried in mud or on the under surface of stones, in pools where darkness and dampness may be ensured till the return of the tide. It is, however, by no means easy to detect and remove it from the encrusting Polyzoa, Ascidians, or Sponges with which it is usually associated. The flat, soft, unsegmented body is so closely appressed to the substratum that its presence is usually only betrayed by its movement, an even gliding motion of the mobile body, which suggested the apt name "la pellicule animée" to Dicquemare. The creeping surface is called ventral, the upper one dorsal, and as the broader end of the body always goes first, it is anterior as opposed to the more pointed posterior extremity. With a lens the characters shown in Figs. 1 and 2 may be observed. The eyes are seen as black dots near the anterior end, and are placed at the sides of a clear oval space, the brain. Along the transparent margin of the body, the ends of the intestinal branches may be seen. These ramify from a lobed stomach or main-gut, and should the specimen be mature, the "uterus" loadedwith eggs forms a dark margin round the latter (Figs. 1 and 2,ut). The ventral surface is whitish, and through it the "pharynx," a frilled protrusible structure, may be dimly observed. The "mouth,"[16]through which the pharynx at the time of feeding is thrust out (Fig. 2,mo), is almost in the centre of the ventral surface. Behind this, a white, V-shaped mark (vd) indicates the ducts of the male reproductive organs, and still further back is the irregular opaque mark of the "shell-gland," by which the egg-shells are formed (Fig. 2, ♀).

fig3Fig. 3.—Leptoplana tremellarisin the act of swimming.A, Seen from the right side during the downward stroke (the resemblance to a skate is striking);B, from above, showing the upward stroke and longitudinal undulations of the swimming lobes;C, side view during the upward stroke;D, transverse sections of the body during the strokes. × 5.

Fig. 3.—Leptoplana tremellarisin the act of swimming.A, Seen from the right side during the downward stroke (the resemblance to a skate is striking);B, from above, showing the upward stroke and longitudinal undulations of the swimming lobes;C, side view during the upward stroke;D, transverse sections of the body during the strokes. × 5.

Leptoplanaemploys two kinds of movement, creeping and swimming. Creeping is a uniform gliding movement, caused by the cilia of the ventral surface, aided perhaps by the longitudinal muscular layers of this surface, and is effected on the under side of the "surface-film" of water almost as well as on a solid substratum. Swimming is a more rapid and elegant movement, employed when alarmed or in pursuit of prey. The expanded fore-parts of the body act as lobes, which are flapped rapidly up over the body and then down beneath it, undulations running rapidly down them from before backwards. The action in fact is somewhat similar to that by which a skate swims, a resemblance pointed out long ago by Dugès[17](Fig. 3).

We have few direct observations on the nature of the food ofLeptoplana, or the exact mode by which it is obtained. Dalyell,[18]who observed this species very carefully, noticed that it was nocturnal and fed upon aNereis, becoming greatly distended and of a green colour after the meal, but pale after a long fast. Keferstein[19]noticed a specimen in the act of devouring aLumbriconereislonger than itself, and also found the radulae ofChitonand Taenioglossate Molluscs in the intestine. That such an apparently weak and defenceless animal does overpower large and healthy Annelids and Mollusca, has not hitherto been definitely proved. Weak or diseased examples may be chiefly selected. The flexibleLeptoplanaadheres firmly to its prey, and the rapid action of the salivary glands of its mobile pharynx quickly softens and disintegrates the internal parts of the victim. The food passes into the stomach (Fig. 2,mg), and is there digested. It is then transferred to the lateral branches of the intestine, and, after all the nutritious matters have been absorbed, the faeces are ejected with a sudden contraction of the whole body through the pharynx into the water.

Leptoplanaprobably does not live more than a year. In the spring or summer, batches of eggs are laid and fixed to algae or stones by one individual, after having been fertilised by another. YoungLeptoplanahatch out in two to three weeks, and lead apelagic existence till they are three or four millimetres in length. In late summer, numbers of such immature examples may be found among sea-weeds andCorallinain tide pools. In the succeeding spring they develop first the male and then the female reproductive organs.

fig4Fig. 4.—Portion of a transverse section ofLeptoplana tremellarisin the hinder part of the body. × 100.bm, Basement (skeletal) membrane;cil, cilia;d.m, diagonal muscles;d.v.m, dorso-ventral muscles;ep, epidermis;f.p, food particles;l.g, lateral intestinal branches cut across;l.m ext, external, andl.m int, internal longitudinal muscle layers;m.c, glandular (mucous) cells;md, their ducts;N, longitudinal nerve;Nu, nuclei of the intestinal epithelium;ov, ovary;ovd, oviduct;par, cells of the parenchyma;r.d, vasa deferentia, with spermatozoa;rm, circular musculature;rh, rhabdites;sh, cells of the shell-gland;te, testes;ve, vasa efferentia;y.c, "yellow cells." (After Lang.)

Fig. 4.—Portion of a transverse section ofLeptoplana tremellarisin the hinder part of the body. × 100.bm, Basement (skeletal) membrane;cil, cilia;d.m, diagonal muscles;d.v.m, dorso-ventral muscles;ep, epidermis;f.p, food particles;l.g, lateral intestinal branches cut across;l.m ext, external, andl.m int, internal longitudinal muscle layers;m.c, glandular (mucous) cells;md, their ducts;N, longitudinal nerve;Nu, nuclei of the intestinal epithelium;ov, ovary;ovd, oviduct;par, cells of the parenchyma;r.d, vasa deferentia, with spermatozoa;rm, circular musculature;rh, rhabdites;sh, cells of the shell-gland;te, testes;ve, vasa efferentia;y.c, "yellow cells." (After Lang.)

Anatomy of Leptoplana tremellaris.—Leptoplanamay be divided into corresponding halves only by a median vertical longitudinal plane. The body and all the systems of organs are strictly bilaterally symmetrical. Excepting the cavities of the organs themselves, the body is solid. A connective "parenchyma" (Fig. 4,par) knits the various internal organs together, while it allows free play of one part on another. These organs are enclosed in a muscular body-wall, clothed externally by the ciliated epidermis, which is separated from the underlying musculature by a strong membrane (Fig. 4,bm), the only skeletal element in the body.

Body-Wall.—The epidermis (Fig. 4,ep) is composed of a single layer of ciliated cells, containing small, highly refractive, pointed rods or "rhabdites" (rh), and gives rise to deeply-placed mucous cells (m.c), which are glandular and pour out on the surface of the body a fluid in which the cilia vibrate. The tenacious hold on a stone whichLeptoplanaexerts if suddenly disturbed, or when grasping its prey, is probably due to the increased glutinous secretion of these glands, aided perhaps by rhabdites, which on such occasions are shot out in great numbers. The basement membrane is an elastic skeletal membrane composed of stellate cells embedded in a firm matrix. It serves chiefly for the origin and insertion of the dorso-ventral muscles (d.v.m). Under the basement membrane lies a very thin layer of transverse muscular fibres (Fig. 4,rm), which are, however, apparently absent on the ventral surface. Then follows a stout layer of longitudinal fibres (l.m ext), and beneath this a diagonal layer (d.m), the fibres of which intersect along the median line in such a way that the inner fibres of one side become the outer diagonal fibres of the other. Lastly, within this again, on the ventral surface, is a second stout longitudinal layer (l.m int). The sucker (sc, Figs. 2 and 5) is a modification of the body-wall at that point. In addition to the dorso-ventral muscles, there exists a complex visceral musculature regulating the movements of the pharynx, intestine, and copulatory organs.

Parenchyma.—The spaces between the main organs of the body are filled by a tissue containing various kinds of cells, salivary glands, shell-glands, and prostate glands. Besides these, however, we find a vacuolated, nucleated, thick-walled network, and to this the word parenchyma is properly applied. Besides its connective function, the parenchyma confers that elasticity on the body whichLeptoplanapossesses in such a high degree. Pigment cells are found in the parenchyma in many Polyclads.

Digestive System.—The general arrangement of this system may be seen in Figs. 2, 5, and 7; and may be compared, especially when the pharynx is protruded, as in Fig. 2, with the gastral system of a Medusa. The "mouth" (there is no anus) is placed almost in the centre of the ventral surface. It leads (Fig. 7, B,phs) into a chamber (the peripharyngeal space) divided into an upper and a lower division by the insertion of a muscular collar-fold (the pharynx,ph), which may be protruded, its free lipsadvancing, through the mouth (Fig. 2), and is then capable of enclosing by its mobile frilled margin, prey as large asLeptoplanaitself. The upper division of the chamber communicates by a hole in its roof[20](the true mouth, Figs. 5 and 7,g.m) with the cavity of the main-gut or stomach (m.g), which runs almost the length of the body in the middle line, forwards over the brain (Fig. 5,up). Seven pairs of lateral gut-branches convey the digested food to the various organs, not directly however, but only after the food mixed with sea-water has been repeatedly driven by peristalsis first towards the blind end of the gut-branches and then back towards the stomach. Respiration is probably largely effected by this means. The epithelium of the intestine (Fig. 4,l.g) of a starving specimen is composed of separate flagellated cells frequently containing "yellow cells."[21]After a meal, however, the cell outlines are invisible. Gregarines, encysted Cercariae, and Orthonectida[22]occur parasitically in the gut-branches.

Anexcretory systemof "flame-cells" and fine vessels has hitherto been seen only by Schultze[23]in this species, which will not, however, resist intact the compression necessary to enable the details to be determined. They are probably similar to those ofThysanozoondescribed on p.25.

Nervous System.—The brain, which is enclosed in a tough capsule (Fig. 5,br), is placed in front of the pharynx, but some distance behind the anterior margin of the body. It is of an oval shape, subdivided superficially into right and left halves by a shallow depression, and is provided in front with a pair of granular-looking appendages, composed of ganglion-cells from which numerous sensory nerves arise, supplying the eyes and anterior region. Posteriorly the brain gives rise to a chiefly motor, nervous sheath (Fig. 5,nn), which invests the body just within the musculature. This sheath is thickened along two ventral lines (Fig. 5,ln) and two lateral lines (n.s), but is very slightly developed on the dorsal surface. Ganglion-cells occur on the course of the nerves, and are particularly large at the point of origin of the great motor nerves.

fig5Fig. 5.—Diagrammatic view of the structure ofLeptoplana tremellarisas a type of the Polycladida. The body is cut across the middle to show the relative position of organs in transverse section. In the posterior half the alimentary canal has been bisected and removed from the left side, to exhibit the deeply placed nervous sheath (nn) and the male reproductive organs.br, Brain;dp, "diaphragm";e, cerebral group of eyes;et, tentacular eye-group;gr, marginal groove;gm, true mouth;lg, lateral gut-branch;ln, longitudinal nerve stem;m, external mouth;mg,mg', main-gut, whole, and bisected;n, sensory nerve supplying the eyes;nn, nervous network lying on the ventral musculature;n.s, lateral nerve;od, oviduct;ov, ovary;pe, penis (in section);ph, pharynx;pr, prostate or "granule gland";sc, sucker;sg, shell-gland;te, testes;up, anterior unpaired gut-branch;ut, uterus;va, vagina (in section);vd, vas deferens;ve, vasa efferentia; ♂, male genital pore; ♀, female pore.

Fig. 5.—Diagrammatic view of the structure ofLeptoplana tremellarisas a type of the Polycladida. The body is cut across the middle to show the relative position of organs in transverse section. In the posterior half the alimentary canal has been bisected and removed from the left side, to exhibit the deeply placed nervous sheath (nn) and the male reproductive organs.br, Brain;dp, "diaphragm";e, cerebral group of eyes;et, tentacular eye-group;gr, marginal groove;gm, true mouth;lg, lateral gut-branch;ln, longitudinal nerve stem;m, external mouth;mg,mg', main-gut, whole, and bisected;n, sensory nerve supplying the eyes;nn, nervous network lying on the ventral musculature;n.s, lateral nerve;od, oviduct;ov, ovary;pe, penis (in section);ph, pharynx;pr, prostate or "granule gland";sc, sucker;sg, shell-gland;te, testes;up, anterior unpaired gut-branch;ut, uterus;va, vagina (in section);vd, vas deferens;ve, vasa efferentia; ♂, male genital pore; ♀, female pore.

Sense Organs.—Leptoplanapossesses eyes, stiff tactile, marginal cilia, and possibly a sense organ in the "marginal groove." The eyes, which are easily seen as collections of black dots lying at the sides of the brain, may be divided into two paired groups: (1) cerebral eyes (Fig. 5,e), and (2) tentacle eyes (et), which indicate the position of a pair of tentacles in allied forms (Fig. 8, A,tand B). Each ocellus consists of a capsule placed at right angles to the surface of the body in the parenchyma, below the dorsal muscles, and with its convex face outwards. It is a single cell in which pigment granules have accumulated. The light, however, can only reach the refractive rods, which lie within it, obliquely at their outer ends. These rods are in connexion with the retinal cells, and thus communicate by the optic nerve with the brain. The cerebral eyes are really paired, and are directed some upwards, some sideways, some downwards.

The "marginal groove" is a shallow depression of the epidermis (Fig. 5,gr) lined by cilia, and containing the ducts of very numerous gland-cells. It runs almost parallel to the anterior margin of the body, a short distance from it, but we have no observations on its functions.

fig6Fig. 6.—Diagram of an eye ofLeptoplanafrom the tentacle group. × 600. (After Lang.)

Fig. 6.—Diagram of an eye ofLeptoplanafrom the tentacle group. × 600. (After Lang.)

Reproductive Organs.—Leptoplanais hermaphrodite, and, as in most hermaphrodites, the reproductive organs are complicated. The male organs are the first to ripen, but this does not appear to prevent an overlapping of the periods of maturity of the male and female products, so that when the eggs are being laid, the male organs are, apparently, still in a functional state. The principal parts are seen in Fig. 5. The very numeroustestes(te) are placed ventrally, and are connected with fine vasaefferentia(ve), which form a delicate network opening at various points into the twovasa deferentia(vd). These tubes, especially when distended with spermatozoa, may easily be seen (Fig. 2,vd) converging at the base of the penis, and connected posteriorly by a loop that runs behind the female genital pore (Fig. 5). Thepenis(pe) is pyriform and muscular, and is divided into two chambers, a large upper one for the spermatozoa, and a smaller lower one for the secretion of a special"prostate" gland. The apex of the penis is eversible and not merely protrusible, being turned inside out when evaginated. Theovaries(Fig. 5,ov) are numerous and somewhat spherical. They are dorsally placed, but when fully developed extend deeply wherever they can find room to do so, and they not only furnish the ova, but elaborate food-yolk in the ova, as there are no special yolk-glands. The slenderoviducts(od) open at several points into the "uterus" (ut) (a misnomer, as no development takes place within it), which encircles the pharynx, and opens by a single duct into thevagina(va). Here the ova are probably fertilised, and one by one invested by theshell-gland(sg) with a secretion which hardens and forms a resistant shell. They are then laid in plate-like masses which are attached to stones or shells. The development is a direct one, and the youngLeptoplana, which hatches in about three weeks, has the outline of a spherical triangle, and possesses most of the organs of the adult. After leading a floating life for a few weeks it probably attains maturity in about nine months.

Classification, Habits, and Structure of the Polycladida.

The Polyclads were so called by Lang on account of the numerous primary branches of their intestine. They are free-living, purely marine Platyhelminthes, possessing multiple ovaries, distinct male and female genital pores (Digonopora), but no yolk-glands. The eggs are small, and in many cases give rise to a distinct larval form, known as "Müller's larva" (Fig. 12). The Polyclads, with one exception,[24]fall into two sub-groups, Acotylea and Cotylea:—

Fig. 8 shows that, starting with a member (A, D) of each division, in which the mouth is almost in the middle of the ventral surface, and the brain and sense organs somewhat remote from the anterior end, we find in the Acotylea a series leading to an elongated form (Cestoplanidae), in which the mouth, pharynx, and genital pores are far back near the hinder end of the body; while in the Cotylea the series leads similarly to the elongated Prosthiostomatidae, in which, however, the pharynx and external apertures are in the front part of the body. This view of the morphology of the Polyclads is due to Lang, and is based on the assumption that the more radially-constructed forms (Fig. 8, A, D) are the primitive ones.

fig7Fig. 7.—Diagrammatic vertical longitudinal sections:A, OfProsthiostomum(type of Cotylea);B, ofLeptoplana;C, ofCestoplana(types of Acotylea). (After Lang.) These figures illustrate the changes which follow the shifting of the mouth from a central position (B) to either end of the body.br, Brain;dphm, "diaphragm";gm, true mouth;lg, openings of lateral gut-branches;m, mouth;mg, main-gut or stomach;mgbr, median gut-branch;ph, pharynx;ph.m, aperture in pharyngeal fold;phs, peripharyngeal sheath;sc, sucker; ♂, male, and ♀, female, genital aperture.

Fig. 7.—Diagrammatic vertical longitudinal sections:A, OfProsthiostomum(type of Cotylea);B, ofLeptoplana;C, ofCestoplana(types of Acotylea). (After Lang.) These figures illustrate the changes which follow the shifting of the mouth from a central position (B) to either end of the body.br, Brain;dphm, "diaphragm";gm, true mouth;lg, openings of lateral gut-branches;m, mouth;mg, main-gut or stomach;mgbr, median gut-branch;ph, pharynx;ph.m, aperture in pharyngeal fold;phs, peripharyngeal sheath;sc, sucker; ♂, male, and ♀, female, genital aperture.

fig8Fig. 8.—Chief forms of Polycladida:A-C, Acotylea;D-F, Cotylea.A,Planocera graffiiLang, nat. size;B,Stylochoplana maculataStimps, × 7;C,Cestoplana rubrocinctaLang, ×4⁄3;D,Anonymus virilisLang, × 3, ventral surface;E,Thysanozoon brocchiiGrube, nat. size; the head is thrown back and the pharynx (ph) is protruded.F,Prosthiostomum siphunculusLang, × 3.Br, Brain;CG, cerebral eye group;DM, true mouth;Ey, marginal eyes;m, mouth;MG, main-gut or stomach;P, dorsal papillae;Ph, pharynx;s, sucker (ventral);T, tentacles;UP, dorsal median gut-branch. ♂, male, and ♀, female, genital aperture, except inD, where ♂ refers to the multiple penes. (After Lang and Schmidt.)

Fig. 8.—Chief forms of Polycladida:A-C, Acotylea;D-F, Cotylea.A,Planocera graffiiLang, nat. size;B,Stylochoplana maculataStimps, × 7;C,Cestoplana rubrocinctaLang, ×4⁄3;D,Anonymus virilisLang, × 3, ventral surface;E,Thysanozoon brocchiiGrube, nat. size; the head is thrown back and the pharynx (ph) is protruded.F,Prosthiostomum siphunculusLang, × 3.Br, Brain;CG, cerebral eye group;DM, true mouth;Ey, marginal eyes;m, mouth;MG, main-gut or stomach;P, dorsal papillae;Ph, pharynx;s, sucker (ventral);T, tentacles;UP, dorsal median gut-branch. ♂, male, and ♀, female, genital aperture, except inD, where ♂ refers to the multiple penes. (After Lang and Schmidt.)

Classification of Polycladida.

Planoceridae.

With dorsal tentacles. Mouth sub-central.

Planocera(Fig. 8, A).

Imogine.

Conoceros.

Stylochus.

Stylochoplana(Fig. 8, B).

Diplonchus.

Planctoplana.

Planocera foliumGrube. Berwick-on-Tweed.

Stylochoplana maculataQuatref. Among brown weeds in Laminarian zone.

Leptoplanidae.

Without dorsal tentacles. Penis directed backwards.

Discocelis.

Cryptocelis.

Leptoplana.

Trigonoporus.

?Polypostia(see p.27).

Leptoplana tremellarisO. F. Müll.

L. fallaxQuatref. Plymouth.

L. droebachensisOe. Plymouth Sound.

L. atomataO. F. Müll. Doubtful species.

Cestoplanidae.

No tentacles. Body elongated. Penis directed forwards.

Cestoplana(Fig. 8, C).

In Mediterranean and on French side of the Channel.

Enantiidae.

No sucker. No tentacles. Main-gut very short. External apertures as inEuryleptidae.

Enantia.

Adriatic Sea.

Anonymidae.

Mouth central. No tentacles. With two rows of penes.

Anonymus(Fig. 8, D).

Naples (two specimens).

Pseudoceridae.

Marginal tentacles folded. Mouth in anterior half.

Thysanozoon(Fig. 8, E).

Pseudoceros.

Yungia.

Euryleptidae.

Tentacles usually present and pointed, or represented by two groups of eyes. Mouth close to anterior end. Pharynx cylindrical.

Prostheceraeus.

Cycloporus.

Eurylepta.

Oligocladus.

Stylostomum.

Aceros.

Prostheceraeus vittatusMont. On west coast.

P. argusQuatref. Guernsey.

Cycloporus papillosusLang. On Ascidians in 2-30 fms.

Eurylepta cornutaO.F. Müll. On sponges and shells, 2-10 fms.

Oligocladus sanguinolentusQuatref.

O. auritusClap. Doubtful.

Stylostomum variabileLang.

Prosthiostomatidae.

Tentacles absent. Body elongated. Pharynx long, cylindrical. Penis with accessory muscular vesicles.

Appearance and Size of Polyclad Turbellaria.—Polyclads are almost unique amongst animals in possessing a broad and thin, delicate body that glides like a living pellicle over stones and weeds, moulding itself on to any inequalities of the surface over which it is travelling, yet so fragile that a touch of the finger will rend its tissues and often cause its speedy dissolution. The dorsal surface in a few forms is raised into fine processes (Planocera villosa), or into hollow papillae (Thysanozoon brocchii), and in very rare cases may be armed with spines (Acanthozoon armatum,[26]Enantia spinifera); in others, again, nettle-cells (nematocysts) are found (Stylochoplana tarda,Anonymus virilis). Some Polyclads, especially the pelagic forms, are almost transparent; in others, the colour may be an intense orange or velvety black, and is then due to peculiar deposits in the epidermal cells. Between these two extremes the colour is dependent upon the blending of two sources, the pigment of the body itself and the tint of the food. Thus a starvedLeptoplanais almost or quite white, a specimen fed on vascular tissue reddish. Many forms are coloured in such a way as to make their detection exceedingly difficult, but this is probably not merely due, as Dalyell supposed, to the substratum furnishing them with food and thus colouring them sympathetically, but is probably a result of natural selection.

The largest Polyclad, the bulkiest Turbellarian, isLeptoplana gigas(6 inches long and 4 in breadth), taken by Schmarda, free-swimming, off the coast of Ceylon. The largest European form isPseudoceros maximus, 3½ inches in length and stoutly built. A British species,Prostheceraeus vittatus, attains a length of from 2 to 3 inches. These large forms, especially the Pseudoceridae (pre-eminently the family of big Polyclads), are brightly coloured, and usually possess good swimming powers, since, being broad and flat, they are certainly not well adapted for creeping rapidly, and this is well shown by the way these Polyclads take to swimming when in pursuit of prey at night. The size of any individual is determined, amongst other factors, by the period at which maturity sets in, after which probably no increase takes place. Polyclads apparently live about twelve months, and mature specimens of the same species vary from ½ inch to 2½ inches in length (Thysanozoon brocchii),showing that growth is, under favourable conditions, very rapid.

Habits of Polyclad Turbellaria.—Polyclads are exclusively marine, and for the most part littoral, animals. Moreover, there is no evidence of their occurrence in those inland seas where certain marine animals (including one or two species of otherwise characteristically marine Rhabdocoelida, p.46) have persisted under changed conditions. From half-tide mark down to 50 fathoms, some Polyclads probably occur on all coasts, but as to their relative abundance in different seas we have very little accurate information. The southern seas of Europe possess more individuals and species than the northern, and probably the maximum development of the group takes place on the coasts and coral islands of the tropics.[27]No Polyclads have been taken below 60 fathoms; but their delicacy and inconspicuousness render this negative evidence of little value. Six truly pelagic forms, however, are known,[28]and these are interesting on account of their wide distribution (three occurring in the Atlantic, Pacific, and Indian oceans), and also from the distinct modifications they have undergone in relation to their pelagic existence.

Whatever may be the interpretations of the fact, Polyclads are notoriously difficult to detect, and this fact doubtless explains the scanty references to them by the older naturalists who collected even in tropical seas. Lang, who worked seven years at Naples, added to the Mediterranean fauna as many Polyclads as were previously known for all Europe, in spite of the assiduous labours of his predecessors, Delle Chiaje and Quatrefages. Again Hallez, collecting at Wimereux at low-water, obtained some twenty specimens ofLeptoplana tremellarisin an hour, while some other collectors working by his side could only find two or three. Yet, even making allowance for the difficulty of finding Polyclads, few of them appear to be abundant.

Leptoplana tremellarisis frequently associated with colonies ofBotryllus, and if separated soon perishes, whereas the free-living individuals are distinctly hardy (Hallez). A closely allied but possibly distinct form lives upon the surface of the PolyzoonSchizoporella, on the French side of the Channel, and cannot long endure separation from its natural habitat, to which it is adaptively coloured. A striking case of protective mimicry is exhibited byCycloporus papillosus, on the British coasts. This species, eminently variable in colour and in the presence or absence of dorsal papillae, is usually a quarter of an inch in length and of a firm consistency. Fixed by its sucker to Polyclinid and other Ascidians,Cycloporusappears part and parcel of the substratum, an interesting parallel toLamellaria perspicua,[29]though we are not justified in calling the Polyclad parasitic. Indeed, though a few cases of association between Polyclads and large Gasteropods, Holothurians, and Echinids are known,[30]there is only one case, that ofPlanocera inquilina,[31]in the branchial chamber of the GasteropodSycotypus canaliculatus, which would seem to bear the interpretation of parasitism. The jet-blackPseudoceros velutinusand the orangeYungia aurantiacaof the Mediterranean, are large conspicuous forms with no attempt at concealment, but their taste, which is not known, may protect them. Other habits, curiously analogous with devices employed by Nudibranch Mollusca (compareThysanozoon brocchiiwithAeolis papillosa), emphasise the conclusion that the struggle for existence in the littoral zone has adapted almost each Polyclad to its particular habitat.

As regards the vertical distribution of this group on the British coasts,Leptoplana tremellarishas an extensive range, and appears to come from deeper to shallower water to breed.[32]In the upper part of the Laminarian zone,Cycloporus papillosus, and, among brown weeds,Stylochoplana maculataare found. At and below lowest water-markProstheceraeus vittatus,P. argus, andEurylepta cornutaoccur.Stylostomum variabileandOligocladus sanguinolentus, though occasionally found between tide-marks, especially in the Channel Islands, are characteristic, along withLeptoplana droebachensisandL. fallax, of dredge material from 10 to 20 fathoms.

Locomotion.—Locomotion is generally performed by Polyclads at night when in search of food, and two methods, creepingand swimming, are usually employed—creeping by the cilia, aided possibly, as in the case of some Gasteropod Mollusca, by the longitudinal muscles of the ventral surface; and swimming, by undulations of the expanded margins of the body. In the former case the cilia work in a glandular secretion which bathes the body, and enables them to effect their purpose equally well on different substrata. The anterior region is generally lifted up, exploring the surroundings by the aid of the tentacles, which are here usually present. The rest of the body is closely appressed to the ground.

Swimming is particularly well performed by the Pseudoceridae, certain species ofProstheceraeus, the large Planoceridae, someStylochoplana,Discocelis, andLeptoplana, and in the same manner as inLeptoplana tremellaris(p.9). InCryptocelis,Leptoplana alcinoi, andL. pallida, however, the whole body executes serpentine movements like an active leech (e.g.Nephelis); a cross section of the body would thus present the same appearance during the whole movement. Many Polyclads, notablyAnonymus(Lang), if irritated, spread out in all directions, becoming exceeding thin and transparent.

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