fig9Fig. 9.—Discocelis lichenoidesMert. (after Mertens), creeping on the inner side of a glass vessel by means of the lobes of the extended and exceedingly mobile pharynx (ph). These lobes also serve to encloseCrustacea(a), and one lobe may then be withdrawn independently of the rest, back into the body (b). The brain (br) and shell-gland (sg) are shown by transparency.
Fig. 9.—Discocelis lichenoidesMert. (after Mertens), creeping on the inner side of a glass vessel by means of the lobes of the extended and exceedingly mobile pharynx (ph). These lobes also serve to encloseCrustacea(a), and one lobe may then be withdrawn independently of the rest, back into the body (b). The brain (br) and shell-gland (sg) are shown by transparency.
Fig. 9.—Discocelis lichenoidesMert. (after Mertens), creeping on the inner side of a glass vessel by means of the lobes of the extended and exceedingly mobile pharynx (ph). These lobes also serve to encloseCrustacea(a), and one lobe may then be withdrawn independently of the rest, back into the body (b). The brain (br) and shell-gland (sg) are shown by transparency.
Discocelis lichenoides,Planocera graffii, andAnonymus virilishave peculiar modes of progression. The first, according to Mertens, will climb up the sides of a vessel by means of the expanded lobes of the pharynx (Fig. 9,ph), a habit of considerable interest, since we know that certain Ctenophores—Lampetia, for instance—progress when not swimming on the expanded lobes of their "stomach."[33]PlanoceraandAnonymuscreep by extending parts of the anterior margin and dragging the rest of the body behind. In consequence, the brain and dorsal tentacles may come to lie actually behind the middle of the body, and thus no definite anterior end or "head" advances first. Along with this curious habit it may be noticed (Lang) that the radial symmetry of the body is well marked; but even without accepting this author's suggestion of the concurrent development of a "head" with locomotion in a definite direction, the facts, whether these two forms are primitive or not, are highly interesting.
Food.—Though we are probably right in calling Polyclads a carnivorous group, the food of very few forms has been ascertained. Those which possess a large frilled pharynx (most Acotylea) probably enclose and digest large, and, it may be, powerful prey, as appears to be the case inLeptoplana tremellaris.Cryptocelis albahas been seen by Lang with the pharynx so distended, owing to a largeDrepanophorus(Nemertine) which it contained, as to resemble a yolk-sac projecting from the under surface of an embryo. The Cotylea such asThysanozoon, with a bell- or trumpet-shaped pharynx, are fond of fixing this to the side of the aquarium, but whether they thus obtain minute organisms is not clear.Prosthiostomumshoots out its long pharynx with great vehemence (Fig. 8, F) and snaps up small Annelids by its aid (Lang). Those Polyclads which, asCycloporusand others, are definitely associated with other organisms are not certainly known to feed upon the latter, though "Planaria velellae" has been seen by Lesson[34]devouring the fleshy parts of its host. The salivary glands which open on the lips and the inner surface of the pharynx powerfully disintegrate the flesh of the prey. Digestion takes place in the main-gut, and the circulation of the food is accomplished by the sphinctral musculature of the intestinal branches (conf.Leptoplana, p. 13).
fig10Fig. 10.—Diagram of the musculature, causing peristaltic movements of the intestinal branches of Polyclads. (After Lang.)
Fig. 10.—Diagram of the musculature, causing peristaltic movements of the intestinal branches of Polyclads. (After Lang.)
Fig. 10.—Diagram of the musculature, causing peristaltic movements of the intestinal branches of Polyclads. (After Lang.)
A distinct vent or anus is always absent. After a meal thefaecal matter collects in the main-gut, and is discharged violently by the pharynx into the water. In a few species, however, the intestinal branches open to the exterior (Lang).Yungia aurantiaca, a large and abundant Neapolitan form, possesses such openings over the greater part of the dorsal surface;Cycloporus papillosushas marginal pores;Oligocladus sanguinolentusapparently possesses an opening at the posterior end of the main-gut; andThysanozoon brocchiifrequently rends at this point, in consequence of the accumulation of food.
Respiration.—The oxygen of the atmosphere dissolved in the sea-water is, in default of a special circulatory fluid, brought to the tissues of Polyclads in two ways. The ciliated epidermis provides a constant change of the surrounding water, by which the superficial organs may obtain their supply; and the peristaltic movements of the digestive system, aided by the cilia of the endoderm cells, ensure a rough circulation of the sea-water, which enters along with the food, to the internal organs. The papillae ofThysanozoon brocchii, containing outgrowths of the intestinal branches, are possibly so much additional respiratory surface, although still larger forms (other Pseudoceridae) are devoid of such outgrowths.
Excretion.—The excretory system of only one Polyclad (Thysanozoon brocchii) is accurately known. Lang, by compressing light-coloured specimens, found the three parts of the system known to occur in many Platyhelminthes: (1) the larger longitudinal canals, and (2) the capillary vessels, which commence with (3) the flame-cells in the parenchyma of the body. The mode of distribution of these parts is not, however, ascertained. The canals are delicate, sinuous, apparently intracellular tubes, coursing close to the margin of the body and sending offsets which suspend the canals to the dorsal surface, where possibly openings may occur. In dilatations of these vessels bunches of cilia, and occasionally flame-cells, are found. Usually, however, flame-cells occur at the commencement or during the course of the capillaries, which are straight, rarely branching, tubes of exceeding tenuity, and appear (Lang) to be outgrowths of the flame-cells, just as the duct is an outgrowth of a gland-cell. In fact there is little doubt that the stellate flame-cells are modified parenchymatous gland-cells, containing a lumen filled with a fluid into which a number of cilia project and vibrate synchronously. The cells excretenitrogeneous waste substances, which are then discharged into the capillaries, whence the cilia of the main vessels drive them presumably to the exterior, though external openings of the excretory system are not known. Traces of this system have been observed in youngLeptoplana(first by Schultze in 1854) and also inCestoplana.
Sensation.—A nervous sheath, with scattered ganglion cells, everywhere underlies the musculature. It is exceedingly faintly marked on the dorsal surface, but laterally and ventrally forms a dense network with polygonal meshes. Thickenings of this sheath give rise to lateral nerves, and also to a pair of stout longitudinal nerves from which the internal organs are probably innervated. The brain, hardly distinct in pelagic Polyclads, in most forms does not differ greatly from that ofLeptoplana(p.13).
The sense organs of Polyclads have the form of tentacles, eyes, otocysts (inLeptoplana otophora), and stiff tactile cilia. The solid dorsal tentacles of Planoceridae contrast strongly with the folded or pointed hollow processes of the Cotylea. The former (Fig. 8, A,T) are muscular and very contractile, and are placed near the brain some distance from the anterior end. The latter are outgrowths of the front margin of the body, and are sometimes (Yungia) provided superficially with olfactory pits and internally with eyes and intestinal coeca.
The eyes which occur in Polyclads may be divided into (a) a pair of cerebral groups overlying the brain; (b) those embedded in the tentacles (tentacular group); and (c) the marginal eyes, which inAnonymusoccur all round the margin. A complex form is sometimes assumed by the cerebral eyes of Pseudoceridae, resulting probably from incomplete fission (Fig. 11).Leptoplana otophorawas obtained by Schmarda on the south coast of Ceylon. On each side of the brain is a capsule containing two otoliths. This is the only known case of the occurrence of these organs in Polyclads.
Reproduction.—Although Polyclads are able to repair the result of injuries to a very considerable extent, they are not known to multiply asexually. The two processes are intimately associated, but, though probably all Turbellaria can regenerate certain lost parts, asexual reproduction only occurs sporadically.
fig11Fig. 11.—Double eye from the cerebral group ofPseudoceros maximus. (After Lang.)
Fig. 11.—Double eye from the cerebral group ofPseudoceros maximus. (After Lang.)
Fig. 11.—Double eye from the cerebral group ofPseudoceros maximus. (After Lang.)
All known Polyclads are hermaphrodite. The male organs, scattered, like the testes ofLeptoplana, over the ventral surface, develop earlier than the ovaries, though the periods of maturation overlap; hence the possibility of self-fertilisation, though remote, is still worth consideration. The genital apertures, through which, in the male, spermatozoa, and in the female, ova, are emitted, are usually situated as inLeptoplana(Figs. 2 and 5, ♂ and ♀). InTrigonoporus, a genus once found at Naples, a secondary female aperture has been discovered leading into the female genital canal[35]; and inAnonymus,Polypostia, andThysanozoon(Fig. 7, E, ♂) two or more male pores and penes have been found.Anonymushas several penes (Fig. 7, D, ♂) arranged radially round the body.Polypostia, a remarkable form described by Bergendal,[36]belonging to the Acotylea, possesses about twenty such structures ranged round the female genital aperture. Lang, whose attention was attracted by these singular facts, made the interesting discovery thatThysanozoonuses its penes as weapons of offence rather than as copulating organs, burying them in the skin of another Polyclad (Yungia) that happened to cross its path, spermatozoa being of course left in the wound. Lang further found thatProstheceraeus albocinctusandCryptocelis albain this way implanted a spermatophore in the skin of another individual of the same species, and he suggested that from this point the spermatozoa wandered through the tissues till they met with and fertilised the eggs. It is now known that a similar process of "hypodermic impregnation" occurs sporadically in several groups of animals.[37]Nevertheless, in some Polyclads it is probable, and inStylochus neapolitanusit is certain, that normal copulation takes place. The sperm-masses are transferred to a coecal diverticulum of the female genital canal, and then by a delicate mechanism, of which we know only the effects, one spermatozoon obtains entrance into one matured ovum, which differs from the ova of most Turbellaria in that it contains in its own protoplasm the yolk necessary for the nutrition of the embryo. In other words, there are no special yolk-glands. After fertilisation, the ovum in all Polyclads is coated with a shell formed by the shell-gland, which also secretes a substance uniting the eggs together. They are deposited on stones and shells, either in plate-like masses or in spirals (like those of Nudibranchs).Cryptocelis albalays masses of an annular shape, with two ova in each shell, and buries them in sand.
Development.[38]—The first stages in the embryology of Polyclads appear to be very uniform. They result, in all Cotylea and in certain Planoceridae, in the formation of a Müller's larva (Fig. 12) about a couple of weeks after the eggs are laid. This larva (1-1.8 mm. long), which is modified in the Planoceridae, is distinguished by the presence of a ciliated band, running somewhat transversely round the body, and usually produced into a dorsal, a ventral, and three pairs of lateral processes. When swimming the body is placed as in Fig. 12, and twists round rapidly about its longitudinal axis by means of the strong locomotor cilia placed in transverse rows upon the processes. The cilia of each row vibrate synchronously, and recall the action of the swimming plates of a Ctenophore. It is noteworthy that whereasStylochus pilidiumpasses through a modified or, according to some authors, a primitive larval stage, its near ally,S. neapolitanus, develops directly. MostAcotylea indeed develop directly, and their free-swimming young differ from Müller's larva merely in the absence of the ciliated band and in the mode of swimming.
fig12Fig. 12.—Section through Müller's larva ofThysanozoon brocchii(modified from Lang). The right half is seen from inside. × 150. Semi-diagrammatic.br, Brain;dl, dorsal ciliated lobe;dr, salivary gland-cells of pharynx;e, eye;ep, ciliated epidermis containing rhabdites;mg, stomach or main-gut;mg1, unpaired gut branch over the brain;mo, "mouth" of larva;n,n1, section of nerves;oe, ectodermic pit forming oesophagus of larva;par, parenchyma filling the space between the alimentary tract and the body wall;ph, pharynx lying in the cavity of the peripharyngeal sheath, the nuclei of which are visible;sl1,sl2,sl3, lateral ciliated lobes of the right side;vl, ventral ciliated lobe.
Fig. 12.—Section through Müller's larva ofThysanozoon brocchii(modified from Lang). The right half is seen from inside. × 150. Semi-diagrammatic.br, Brain;dl, dorsal ciliated lobe;dr, salivary gland-cells of pharynx;e, eye;ep, ciliated epidermis containing rhabdites;mg, stomach or main-gut;mg1, unpaired gut branch over the brain;mo, "mouth" of larva;n,n1, section of nerves;oe, ectodermic pit forming oesophagus of larva;par, parenchyma filling the space between the alimentary tract and the body wall;ph, pharynx lying in the cavity of the peripharyngeal sheath, the nuclei of which are visible;sl1,sl2,sl3, lateral ciliated lobes of the right side;vl, ventral ciliated lobe.
Fig. 12.—Section through Müller's larva ofThysanozoon brocchii(modified from Lang). The right half is seen from inside. × 150. Semi-diagrammatic.br, Brain;dl, dorsal ciliated lobe;dr, salivary gland-cells of pharynx;e, eye;ep, ciliated epidermis containing rhabdites;mg, stomach or main-gut;mg1, unpaired gut branch over the brain;mo, "mouth" of larva;n,n1, section of nerves;oe, ectodermic pit forming oesophagus of larva;par, parenchyma filling the space between the alimentary tract and the body wall;ph, pharynx lying in the cavity of the peripharyngeal sheath, the nuclei of which are visible;sl1,sl2,sl3, lateral ciliated lobes of the right side;vl, ventral ciliated lobe.
fig13Fig. 13.—Diagrammatic transverse sections of a larval Polyclad at different stages, to illustrate the development of the pharynx. (After Lang.)A, Larva of the eighth day still within the shell. The main-gut (mg) is still solid, the epidermis is slightly invaginated, and a pair of muscular mesodermic thickenings (ms) are present.B, Young pelagic larva. The epidermic invagination has deepened and developed laterally.C, The lateral pouches have formed the wall of the peripharyngeal sheath, enclosing the mesodermic, muscular, thickening or pharyngeal fold (ph). (Compare Fig. 12.) Towards the end of larval life, when the ciliated processes (sl, Fig. 12) have aborted, the stageDis reached. By the opening outwards of the pharyngeal sheath (ph.sh) the two aperturesgm, or true mouth, andm, or external mouth, are formed, which together correspond with the oesophageal opening of the younger larva. (Compare the transverse section in Fig. 5.)
Fig. 13.—Diagrammatic transverse sections of a larval Polyclad at different stages, to illustrate the development of the pharynx. (After Lang.)A, Larva of the eighth day still within the shell. The main-gut (mg) is still solid, the epidermis is slightly invaginated, and a pair of muscular mesodermic thickenings (ms) are present.B, Young pelagic larva. The epidermic invagination has deepened and developed laterally.C, The lateral pouches have formed the wall of the peripharyngeal sheath, enclosing the mesodermic, muscular, thickening or pharyngeal fold (ph). (Compare Fig. 12.) Towards the end of larval life, when the ciliated processes (sl, Fig. 12) have aborted, the stageDis reached. By the opening outwards of the pharyngeal sheath (ph.sh) the two aperturesgm, or true mouth, andm, or external mouth, are formed, which together correspond with the oesophageal opening of the younger larva. (Compare the transverse section in Fig. 5.)
Fig. 13.—Diagrammatic transverse sections of a larval Polyclad at different stages, to illustrate the development of the pharynx. (After Lang.)A, Larva of the eighth day still within the shell. The main-gut (mg) is still solid, the epidermis is slightly invaginated, and a pair of muscular mesodermic thickenings (ms) are present.B, Young pelagic larva. The epidermic invagination has deepened and developed laterally.C, The lateral pouches have formed the wall of the peripharyngeal sheath, enclosing the mesodermic, muscular, thickening or pharyngeal fold (ph). (Compare Fig. 12.) Towards the end of larval life, when the ciliated processes (sl, Fig. 12) have aborted, the stageDis reached. By the opening outwards of the pharyngeal sheath (ph.sh) the two aperturesgm, or true mouth, andm, or external mouth, are formed, which together correspond with the oesophageal opening of the younger larva. (Compare the transverse section in Fig. 5.)
Polyclads possess an undoubted mesoderm, which gives rise to the muscles, the pharyngeal fold, and the parenchyma. The ectoderm forms the epidermis, in the cells of which therhabdites (Fig. 12) arise, apparently as so many condensed secretions. From the ectoderm the brain arises as two pairs of ingrowths, which fuse together, and from these the peripheral nervous system grows out. Three pigmented ectoderm cells give rise, by division, to the eyes—an unpaired cell (Fig. 12,e) to the cerebral group of eyes, and the other two to the marginal and tentacular groups. The copulatory organs apparently arise to a large extent as ingrowths from the ectoderm, from which the accessory glands (prostates, shell-glands) are also formed. The endoderm forms the lining of the main-gut and its branches. The pharynx is developed as in Fig. 13, which shows that the "mouth" of the young larva (C) does not correspond exactly with that of the adult (D). The salivary glands arise from ectoderm cells, which sink deeply into the parenchyma. The reproductive organs (ovaries and testes) possibly arise by proliferation from the gut-cells (Lang, v. Graff). The change from the larva to the adult is gradual, the ciliary band being absorbed and the creeping mode of life adopted.
Turbellaria. II. Tricladida.
The Triclads are most conveniently divided into three groups[39]: (i.)Paludicola, the Planarians of ponds and streams; (ii.) theMaricola, the Triclads of the sea; and (iii.)Terricolaor Land Planarians. From the Polyclads they differ in their mode of occurrence; in the elongated form of their body and almost constant, mid-ventral position of the mouth; in possessing a single external genital pore (Monogopora); and in the production of a few, large, hard-shelled eggs provided with food-yolk.
Occurrence of the Paludicola.—The Planarians of our ponds and streams are the most familiar and accessible Turbellaria. Their elongated, flattened bodies, and gliding movements, render them conspicuous objects on the under surface of stones and on the leaves of aquatic plants, where they live gregariously. The variablePolycelis nigra(Fig. 14, H) is very abundant in stagnant water and slowly-moving streams, whereas its ally,P. cornuta(Fig. 14, G), distinguished by a pair of tentacles, is more local.Planaria(Dendrocoelum)lactea(A),P. polychroa(I),P. torva, andP. punctataare not infrequently found together, but the last is at once the largest and rarest.
fig14Fig. 14.—Forms of Triclads, with the distinguishing specific characters of certain British forms.A,Planaria lacteaO. F. M., × 2;B,Planaria alpinaDana, × 4 (after Kennel);C,Phagocata gracilisLeidy (after Woodworth), × 6;C', the same with the pharynges (ph) extruded;D,Gunda ulvaeOer., × 4;E,Planaria gonocephalaDug. (after Schmidt), × 4;F, genitalia ofGunda ulvae(after Wendt);G, head ofPolycelis cornutaSchm.;H, head ofPolycelis nigraEhr.;I, head ofPlanaria polychroaSchm.KtoNshow the distinctive characters of the genital ducts inK,Polycelis nigra;L,Planaria polychroa;M,Planaria alpina;N,Planaria torvaSchultze (after Iijima and v. Kennel).ga, Genital atrium;go, common genital opening;mgr, "musculo-glandular organ";mo, "mouth";ovd, oviduct;pe, penis;ph, pharynx;pyr, pyriform organs of unknown significance;sc, sucker;sp, spermatophore lying in (ut) uterus;vd, vesicula seminalis. (All exceptCandEare found in England.)
Fig. 14.—Forms of Triclads, with the distinguishing specific characters of certain British forms.A,Planaria lacteaO. F. M., × 2;B,Planaria alpinaDana, × 4 (after Kennel);C,Phagocata gracilisLeidy (after Woodworth), × 6;C', the same with the pharynges (ph) extruded;D,Gunda ulvaeOer., × 4;E,Planaria gonocephalaDug. (after Schmidt), × 4;F, genitalia ofGunda ulvae(after Wendt);G, head ofPolycelis cornutaSchm.;H, head ofPolycelis nigraEhr.;I, head ofPlanaria polychroaSchm.KtoNshow the distinctive characters of the genital ducts inK,Polycelis nigra;L,Planaria polychroa;M,Planaria alpina;N,Planaria torvaSchultze (after Iijima and v. Kennel).ga, Genital atrium;go, common genital opening;mgr, "musculo-glandular organ";mo, "mouth";ovd, oviduct;pe, penis;ph, pharynx;pyr, pyriform organs of unknown significance;sc, sucker;sp, spermatophore lying in (ut) uterus;vd, vesicula seminalis. (All exceptCandEare found in England.)
Fig. 14.—Forms of Triclads, with the distinguishing specific characters of certain British forms.A,Planaria lacteaO. F. M., × 2;B,Planaria alpinaDana, × 4 (after Kennel);C,Phagocata gracilisLeidy (after Woodworth), × 6;C', the same with the pharynges (ph) extruded;D,Gunda ulvaeOer., × 4;E,Planaria gonocephalaDug. (after Schmidt), × 4;F, genitalia ofGunda ulvae(after Wendt);G, head ofPolycelis cornutaSchm.;H, head ofPolycelis nigraEhr.;I, head ofPlanaria polychroaSchm.KtoNshow the distinctive characters of the genital ducts inK,Polycelis nigra;L,Planaria polychroa;M,Planaria alpina;N,Planaria torvaSchultze (after Iijima and v. Kennel).ga, Genital atrium;go, common genital opening;mgr, "musculo-glandular organ";mo, "mouth";ovd, oviduct;pe, penis;ph, pharynx;pyr, pyriform organs of unknown significance;sc, sucker;sp, spermatophore lying in (ut) uterus;vd, vesicula seminalis. (All exceptCandEare found in England.)
Planaria alpina(Fig. 14, B) is characteristic of cold mountain streams, but occurs down tosea-level in England, the Isle of Man, and Ireland, and from its abundance in spring water, probably enjoys a wide distribution underground. In the Swiss Alps it has been found at altitudes of over 6000 feet, at lower levels in the Rhone, and also in the Lake of Geneva. This wide distribution may perhaps be accounted for, partly, by its faculty for asexual reproduction in summer, and also, by the production, later in the year, of hard-shelled eggs which are laid loosely, not attached to stones or plants.[40]But we have no really direct evidence of the means of dispersal of this or of any of the foregoing species, although they all have a wide distribution in Europe. Of extra-European forms the accounts that exist are very fragmentary. The only indubitable diagnostic character of a Triclad is the structure of its genital ducts, and this is accurately known in only a few cases. Several species such asDicotylus pulvinar(Fig. 16, B), at present known only from Lake Baikal,[41]and others (Planaria mrazekii,P. albissima) from Bohemia,[42]will doubtless be found elsewhere when they are carefully looked for.Phagocata gracilisis a remarkable North American form, possessing several pharynges (Fig. 14, C and C'), recalling the independent movement of the pharyngeal lobes ofDiscocelis lichenoides(Fig. 9).[43]
Occurrence of the Maricola.—Little as we accurately know of the distribution of the fresh-water Planariae, our knowledge of the occurrence of the marine forms is still more limited.Gunda(Procerodes)ulvae(Fig. 14, D) is the commonest European form, occurring abundantly in the upper part of the littoral zone, on the shores of the Baltic.G. segmentatafrom Messina has been carefully described by Lang,[44]but these are almost the only species of Maricola which can be accurately determined. They differ from the Paludicola in the position of the "uterus" behind the genital pore and in the absence of a "musculo-glandular organ" (Fig. 14, F). A special interest attaches to the Bdellouridae, a family containing three species, all parasitic onLimulusfrom the east coast of America. These remarkable Triclads usually have a sucker at the hinder end of the body, by which they attach themselves firmly to the cephalo-thoracic appendages and to thegill-plates, upon which the eggs may be found in considerable numbers. One species,Syncoelidium pellucidum, possesses a pair of problematical organs in the hinder part of the body, opening to the exterior ventro-laterally by a couple of chitinous mouth-pieces, but having no connexion with the genital ducts.[45]
Occurrence and Distribution of Land Planarians.—The terricolous Triclads or Land Planarians are the most interesting division of the group. Some forms, such asBipalium kewense, attain large dimensions, being usually 6 to 9 inches in length, and specimens fully extended have measured 18 inches. Their bodies are frequently banded or striped with brilliant colours.Geoplana coeruleaMos. has a blue ventral surface and is olive green or dark Prussian blue above.G. splendensDendy, is marked dorsally by three stripes of emerald green alternating with four dark brown longitudinal bands. The mode of coloration, though somewhat variable, is an important specific character. Its significance, however, is not clearly understood. The colours may be a warning signal, as someGeoplanaat least are disagreeable to the taste of man and some birds[46]; but since Land Planarians are largely nocturnal animals, living by day under logs, banana leaves, and in other moist and dark situations, this explanation is clearly insufficient. TwoGeoplanahave been noticed by Mr. Dendy which seem to be protectively coloured.G. triangulatavar.australisoccurs abundantly in the beech forest in the South Island of New Zealand, and its brown back and yellow or orange ventral surface match the leaves around its haunts.G. gelatinosaagain looks like a mere slimy patch on the rotten bark where it is found. In arid districts, during the dry season, Land Planarians burrow in the soil and form a cyst, in which they lie coiled up, after the manner of earthworms.[47]The glutinous investment of their delicate bodies forms a moist medium in which the cilia covering the body (and especially the ventral surface) may constantly and evenly vibrate, and by which they adhere firmly to their prey. In some tropical Planarians, in addition to possessing offensive properties, the mucus is so copious in amount and hardens with such rapidity, thatthese Triclads may creep over bridges of it, and may even be blown from one stem or branch of a plant to another, hanging at the ends of their threads.[48]
fig15Fig. 15.—Some Land Planarians found in Europe.A,Bipalium kewenseMos. × ⅓ (after Bergendal);B,Rhynchodemus terrestrisO. F. M., × 2;C,Geodesmus bilineatusMetsch., × 2½ (after Metschnikoff).mr, Region of mouth;gp, region of genital pore.
Fig. 15.—Some Land Planarians found in Europe.A,Bipalium kewenseMos. × ⅓ (after Bergendal);B,Rhynchodemus terrestrisO. F. M., × 2;C,Geodesmus bilineatusMetsch., × 2½ (after Metschnikoff).mr, Region of mouth;gp, region of genital pore.
Fig. 15.—Some Land Planarians found in Europe.A,Bipalium kewenseMos. × ⅓ (after Bergendal);B,Rhynchodemus terrestrisO. F. M., × 2;C,Geodesmus bilineatusMetsch., × 2½ (after Metschnikoff).mr, Region of mouth;gp, region of genital pore.
In Europe there are only two or three indigenous Land Planarians, of whichRhynchodemus terrestrisO. F. M. (Fig. 15, B) is the most widely distributed, and has been found in moist situations for the most part wherever it has been carefully looked for. It measures about ¾ inch in length, and is dark grey above, whitish below, and bears a pair of eyes near the anterior extremity (Fig. 15, B).Bipalium kewense(Fig. 15, A), which has been found in the forests of Upolu, Samoa, by Mr. J. J. Lister, has been accidentally imported, from the (unknown) districts where it is indigenous, with plants and soil to various parts of the world—England, Germany, the Cape, and also to Sydney, where it appears to have established itself. In these Bipalia living in hothouses, the genitalia never appear to attain maturity, and apparently multiple fission and subsequent reparation of the missing parts is the only mode of reproduction.Geodesmus bilineatus(Fig. 15, C), which has occurred at Giessen, Würzburg, and Dresden, has, in all probability, been introduced with ferns from the West or East Indies.Microplana humicola, described by Vejdovsky from dunghills in Bohemia, is doubtfully indigenous.
In marked contrast with the poverty of the temperate zones in Land Planarians, is the abundance and great variety of this group in Southern Asia, South America, and especially in Australasia, where the rich Land Planarian fauna has been carefully investigated by Spencer, Dendy, Fletcher, and others, incertain parts of Victoria, New South Wales, and New Zealand.[49]About forty species of Planarians have been discovered on the Australian continent, thirty-five of which belong to the predominant genusGeoplana, distinguished by the presence of numerous eyes along the border of the simple anterior extremity. Of the remaining five, four belong to the genusRhynchodemus, with, lastly, the introducedBipalium kewense. The distribution of any one species, however, is so limited that only three forms are common to the two former colonies; and although some of the twenty known New Zealand Planarians (chiefly species ofGeoplana), are identical with Australian species, yet only one, or possibly two, varieties of these species are Australian also. In addition to their prevalence in Australasia, theGeoplanidaealso occur in South America, South Africa, Japan, and the East Indies. TheBipaliidaeare characteristic of the Oriental region, being found in China, Borneo, Bengal, and Ceylon. TheRhynchodemidaeare a cosmopolitan family, occurring in Europe, North and South America, the Cape of Good Hope, Ceylon, the East Indies, Australia (particularly Lord Howe Island), and Samoa.[50]
Habits and Structure of Triclads.—The commonPlanaria(Dendrocoelum)lactea, which usually progresses by ciliary action, aided, it is said, by muscular contractions of the ventral surface, performs, if alarmed, a series of rapid "looping" movements, by affixing a sucker (Fig. 14, A,sc), placed on the under side of the head, to the substratum, and pulling the posterior end close to this. The sucker, discovered by Leydig, is even better developed inP. punctata(Fig. 16, A),P. mrazekii, andP. cavatica, and is an efficient adhering-organ which has probably been developed from a similar but simpler structure found in a considerable number of both fresh-water and marine Triclads (P. alpina, Fig. 16, E). Probably the sucker of the Land PlanarianCotyloplana(D) is the same structure, but the two suckers ofDicotylus(B) are at present unique.Planaria dioica, found by Claparède on the coast of Normandy,[51]is covered with minute adhesive papillae,similar to those of certain Rhabdocoelida (e.g.Monotus, Fig. 19, D), enabling it to cling tightly to theZostera, and so to resist the loosening action of the waves.
fig16Fig. 16.—Suckers of Triclads.A,Planaria punctataPall.;a, dorsal surface of head;b, ventral surface (freely moving) showing the sucker;c, sucker contracted (after Hallez):B, ventral surface of head ofDicotylus pulvinarGr., from Lake Baikal (after Grube):C, dorsal surface ofProcotylea fluviatilisGir. (after Girard):D, sucker ofCotyloplana whiteleggeiSp. (after Spencer):E, ventral view of head ofPlanaria alpinaDana (preserved specimen);hg, adhering groove;m, thickened musculature forming the margin of the sucker;sc, sucker;t, tentacles.
Fig. 16.—Suckers of Triclads.A,Planaria punctataPall.;a, dorsal surface of head;b, ventral surface (freely moving) showing the sucker;c, sucker contracted (after Hallez):B, ventral surface of head ofDicotylus pulvinarGr., from Lake Baikal (after Grube):C, dorsal surface ofProcotylea fluviatilisGir. (after Girard):D, sucker ofCotyloplana whiteleggeiSp. (after Spencer):E, ventral view of head ofPlanaria alpinaDana (preserved specimen);hg, adhering groove;m, thickened musculature forming the margin of the sucker;sc, sucker;t, tentacles.
Fig. 16.—Suckers of Triclads.A,Planaria punctataPall.;a, dorsal surface of head;b, ventral surface (freely moving) showing the sucker;c, sucker contracted (after Hallez):B, ventral surface of head ofDicotylus pulvinarGr., from Lake Baikal (after Grube):C, dorsal surface ofProcotylea fluviatilisGir. (after Girard):D, sucker ofCotyloplana whiteleggeiSp. (after Spencer):E, ventral view of head ofPlanaria alpinaDana (preserved specimen);hg, adhering groove;m, thickened musculature forming the margin of the sucker;sc, sucker;t, tentacles.
The movements of Land Planarians are somewhat peculiar. The ventral surface ofBipaliumhas a median groove, into which the ducts of numerous mucus-glands open. This is bordered by two ridges clothed with long and powerful cilia, which perform the chief part in propelling the animal, aided, according to Lehnert,[52]by muscular waves which pass from the head, backwards,i.e.opposite in direction to those by which a snail slides along. This observation, however, needs confirmation. The whole body executes sinuous movements, during which the crescentic head, lifted slightly above the ground (Fig. 15, A), is constantly altering and regaining its normal shape, somewhat as aPlanaria lacteauses the lobes of its head. Further examination shows that the margin of the head ofBipaliumis not only provided with eyes, but in addition, with ciliated, (probably) olfactory pits. Such depressions, innervated directly from the cerebral ganglia, have been found in sixteen species ofGeoplana,and in one or two species ofRhynchodemus.[53]Some Land Planarians (a species ofRhynchodemusfrom Ceylon, and aDolichoplanafrom the Philippines) wriggle out of a box or the hand with great speed (Moseley).
The skin of Triclads is full of minute rods or rhabdites, which are shot out in great numbers when the animal is irritated, and doubtless serve an offensive purpose. The Terricola possess two kinds of these: (1) needle-like rods; and (2) inBipalium kewense, flagellated structures, bent into a V-form and with a slender thread attached to one end (Shipley). InGeoplana coeruleathese bent rods furnish the blue colour of the ventral surface. The rhabdites arise in all Triclads in cells below the basement-membrane, which they are said to traverse in order to reach the epidermis, thus differing in origin, and also in structure, from the rods of Polyclads.
Food.—Triclads are largely if not wholly carnivorous animals, feeding upon Annelids, Crustacea, Insects, Insect-larvae, and Molluscs. The mouth is usually mid-ventral or behind the middle of the body, but in the anomalousLeimacopsis terricolaSchm. from the Andes[54]and inDolichoplanait is near the anterior end. The pharynx (Figs. 17, 18,ph) is cylindrical or bell-shaped, exceedingly dilatable and abundantly supplied with glands and nervous tissue. It opens into the three main intestinal branches, one of which runs in the median plane forwards, the others backwards right and left, enclosing a space in which the genital ducts lie (Figs. 14, A, 17). The fresh-water Planarians prey upon Oligochaeta, Hydrophilidae (aquatic beetles), and the commoner pond-snails.Bipalium kewensepursues earthworms, seizes the upper surface of the anterior end by the glutinous secretion of its ventral surface, and then proceeds to envelop part or the whole of the worm within its pharynx, which is stretched as a thin skin over the body of its struggling prey (Lehnert). The tissues of the latter pass into the intestine of the Planarian, and distend it greatly. After such a meal, which lasts from one to five hours, aBipaliummay remain for three months without seeking food.Geobia subterranea, a white eyeless form from Brazil, pursues earthworms (Lumbricus corethrurus) in their burrows, and has been seen by Fritz Müller sucking the blood out of a youngworm.[55]Geoplana typhlops, a Tasmanian species, is also blind, and pursues worms, as doesG. triangulata(Dendy). In Trinidad, von Kennel[56]observed that land-snails (Subulinae) were the food of certain Land Planarians, the name of which, however, he does not state. The pharynx was employed to suck out the soft parts of the snail even from the upper whorls of the shell.
Reproduction.—InPlanaria lacteathe numerous testes (Fig. 17,te) are placed both above and below the alimentary canal throughout the greater part of its course. The membrane of each gonad is continued into a minute vas efferens, which unites with those of neighbouring testes. Two vasa deferentia (v.d) arise thus on each side, one from the posterior, the other from the anterior testes of the body, and open into the vesiculae seminales (v.s), which may be seen in the living animal as tortuous whitish tubes at the sides of the pharynx (Fig. 14, A). These open into the penis (Figs. 14, A; 17,pe), a large pyriform organ, the apex of which, when retracted, points forwards, projecting into the penial cavity. When this apical portion is evaginated and turned inside out, it is of considerable length, and is able to pass into the long slender duct of the uterus (ut) of another individual. The penial sheath (ps) is part of the genital atrium (gs), which is developed as a pit from the skin, and invests the end of the genital ducts, the mouth of the pit forming the common genital pore (go), through which both male and female genital products are emitted.
There are two ovaries (ov) placed far forwards, between the third and fourth pairs of intestinal coeca. The oviducts (ovd) lie just over the lateral nerves, and have a slightly tortuous course, at each outward bend receiving the duct (yo) of a yolk-gland (yg), so that ova and yolk are already associated when the oviducts open by a short unpaired tube into the genital atrium. The yolk-glands develop rapidly,[57]and when fully formed are massive glands occupying the spaces between the intestinal branches and the testes which are then aborting. The so-calleduterus(ut), apparently at first a diverticulum of the genital atrium, expands behind the pharynx into a receptacle lined by long glandular columnar cells, which, however, are not all of the same kind. The uterine duct opens into the atrium just above the aperture of a problematical, eversible, "musculo-glandular organ" (mgr).
fig17Fig. 17.—Diagrammatic view of the structure ofPlanaria(Dendrocoelum)lactea. × 7. The body has been cut across and a portion removed. In the posterior half the alimentary tract of the left side is removed and the uterus, penis, and muscular organ sliced open horizontally. The nervous system is represented by black, and the yolk-glands by dotted lines.br, Brain;ey, eye with lens and optic nerve;go, external genital aperture for both male and female products;gs, genital atrium;lg, paired lateral intestinal branch;ln, longitudinal nerve;mg, unpaired anterior intestine, the branches of which are cut off close to the main stem;mgr, eversible "musculo-glandular organ";nc, nerve-cells in the pharynx;nn, lateral nerve-twigs;ns, nerve-sheath;ov, ovary;ovd, oviduct;pe, the eversible penis, the corrugated inner white portion of which is the apex;ph, pharynx;phs, pharyngeal sheath;pr, "prostate" or granule-gland (represented by dotted lines opening into the penis);ps, penial sheath;te, testes;to, tactile lobe of the head;ut, "uterus" opening into the genital atrium just abovemgr;vd, vasa deferentia;vs, vesicula seminalis;yg, yolk-glands;yo, openings of the yolk-ducts into the oviducts.
Fig. 17.—Diagrammatic view of the structure ofPlanaria(Dendrocoelum)lactea. × 7. The body has been cut across and a portion removed. In the posterior half the alimentary tract of the left side is removed and the uterus, penis, and muscular organ sliced open horizontally. The nervous system is represented by black, and the yolk-glands by dotted lines.br, Brain;ey, eye with lens and optic nerve;go, external genital aperture for both male and female products;gs, genital atrium;lg, paired lateral intestinal branch;ln, longitudinal nerve;mg, unpaired anterior intestine, the branches of which are cut off close to the main stem;mgr, eversible "musculo-glandular organ";nc, nerve-cells in the pharynx;nn, lateral nerve-twigs;ns, nerve-sheath;ov, ovary;ovd, oviduct;pe, the eversible penis, the corrugated inner white portion of which is the apex;ph, pharynx;phs, pharyngeal sheath;pr, "prostate" or granule-gland (represented by dotted lines opening into the penis);ps, penial sheath;te, testes;to, tactile lobe of the head;ut, "uterus" opening into the genital atrium just abovemgr;vd, vasa deferentia;vs, vesicula seminalis;yg, yolk-glands;yo, openings of the yolk-ducts into the oviducts.
Fig. 17.—Diagrammatic view of the structure ofPlanaria(Dendrocoelum)lactea. × 7. The body has been cut across and a portion removed. In the posterior half the alimentary tract of the left side is removed and the uterus, penis, and muscular organ sliced open horizontally. The nervous system is represented by black, and the yolk-glands by dotted lines.br, Brain;ey, eye with lens and optic nerve;go, external genital aperture for both male and female products;gs, genital atrium;lg, paired lateral intestinal branch;ln, longitudinal nerve;mg, unpaired anterior intestine, the branches of which are cut off close to the main stem;mgr, eversible "musculo-glandular organ";nc, nerve-cells in the pharynx;nn, lateral nerve-twigs;ns, nerve-sheath;ov, ovary;ovd, oviduct;pe, the eversible penis, the corrugated inner white portion of which is the apex;ph, pharynx;phs, pharyngeal sheath;pr, "prostate" or granule-gland (represented by dotted lines opening into the penis);ps, penial sheath;te, testes;to, tactile lobe of the head;ut, "uterus" opening into the genital atrium just abovemgr;vd, vasa deferentia;vs, vesicula seminalis;yg, yolk-glands;yo, openings of the yolk-ducts into the oviducts.
Fertilisation appears to occur in the uterus, where ova, yolk, and spermatozoa, or (inP. torva) spermatophores (Fig. 14, N,sp), are found. The formation of the cocoon inPlanaria lacteais probably begun in the "uterus," but is undoubtedly completed in the genital atrium. InP. polychroa, however, the stalked cocoon is formed wholly in the "uterus." Thus we find two types of cocoons in different species of the genusPlanariaassociated with two types of reproductive organs (Hallez):—
I. Planariae in which the two oviducts open separately into the posterior part of the duct of the uterus. A musculo-glandular organ is absent. The cocoons are spherical and stalked. Examples—Planaria polychroa(Fig. 14, L),P. albissima,P. gonocephala.
II. Planariae in which the two oviducts open by means of an unpaired duct into the genital atrium. A musculo-glandular organ present (Planaria torva(Fig. 14, N),P. mrazekii,P. lactea,P. cavatica), or absent (P. alpina, Fig. 14, M). The cocoons are sessile.
The genitalia of the Maricola (Fig. 14, F) and Terricola do not differ very much from those ofPlanaria. The uterus (greatly reduced in the Land Planarians) lies behind the genital pore, and several ova, together with much milky yolk, are enclosed in a capsule which is formed in the genital atrium.
Asexual Reproduction.[58]—It has long been known that fresh-water Planarians have not only great powers of repairing injuries, but that they use this faculty in order to multiply by transverse fission.Planaria alpinaandPolycelis cornuta, in summer, separate off the posterior part of the body, and this ultimately becomes an entire individual.P. albissima, and especiallyP. subtentaculata, anticipate matters so far, that before fission is complete, the new individual has a head nearly fully formed. The new organs are largely regenerated in both parent and young,apparently by the division and specialisation of scattered embryonic cells in the parenchyma. The asexual reproduction of Land Planarians is not fully proved, though it is known that they repair injuries to the body completely, and thatBipalium kewenseis often found in hothouses, divided into fragments which regenerate all the organs of the parent, but like the latter, do not mature their sexual organs.
fig18Fig. 18.—Semi-diagrammatic view of the excretory system ofPlanaria lactea. (Partly after Chickoff.)can, Capillary network on both dorsal and ventral surfaces;g.br, branches of the intestine;lg, lateral branches of the digestive system;ln, longitudinal nerve;ph, pharynx, with intermuscular capillary excretory network arising from the point markedpht;tp, principal vessels of the excretory system, the external opening of which is not certainly known;vs, vesicula seminalis.
Fig. 18.—Semi-diagrammatic view of the excretory system ofPlanaria lactea. (Partly after Chickoff.)can, Capillary network on both dorsal and ventral surfaces;g.br, branches of the intestine;lg, lateral branches of the digestive system;ln, longitudinal nerve;ph, pharynx, with intermuscular capillary excretory network arising from the point markedpht;tp, principal vessels of the excretory system, the external opening of which is not certainly known;vs, vesicula seminalis.
Fig. 18.—Semi-diagrammatic view of the excretory system ofPlanaria lactea. (Partly after Chickoff.)can, Capillary network on both dorsal and ventral surfaces;g.br, branches of the intestine;lg, lateral branches of the digestive system;ln, longitudinal nerve;ph, pharynx, with intermuscular capillary excretory network arising from the point markedpht;tp, principal vessels of the excretory system, the external opening of which is not certainly known;vs, vesicula seminalis.
Excretion.—The excretory organs of Triclads consist of flame-cells, canaliculi, and a pair of longitudinal canals, the external openings of which, have not been satisfactorily ascertained. The flame-cells are difficult to detect inPlanaria lactea, and the latest observer, Chickoff,[59]was unable to see them, although to him we are indebted for figures of this system inP. lactea(Fig. 18) andP. alpina(P. montana). In the latter, the flame-cells are distinct, and may open directly into the two main canals or indirectly through unbranched canaliculi. The pharynx possesses a special supply of excretory tubules communicating with the main canals. A similar system has been described and figured inGunda segmentataby Lang.[60]
Classification of Tricladida.
Planaria lacteaO. F. M.,P. punctataPall.,P. polychroaSchm.,P. torvaM. Sch.,P. alpinaDana.
Polycelis nigraEhr.,P. cornutaSchm.
Anocelis.
Oligocelis,Procotyla.(Doubtful genera.)
Sorocelis.
Dicotylus.
Procerodes(=Gunda)ulvaeOersted,P. littoralisvan Beneden.
Cercyra.
Uteriporus.
Bdelloura.
Syncoelidium.
Geoplana.
Geodesmus.
Dolichoplana.
Polycladus.
Microplana.
Leimacopsis.
Turbellaria. III. Rhabdocoelida.
The Rhabdocoelida include a very heterogeneous assemblage of usually minute Turbellaria, distinguished collectively from the Polyclads and Triclads by the form of the digestive tract. This is a simple or slightly lobed sac, except in the Bothrioplanidae, which in this and many other points closely resemble the Triclads. It is to the straight, rod-like nature of the alimentary canal that the name of the group refers. The size and form of the body, and the structure of the pharynx and genitalia, vary within wide limits.
The Rhabdocoelida are subdivided into three tribes:—
(1)Acoela, in which a sub-central mouth and pharynx are present, but lead into the parenchyma of the body, not into an intestine with proper walls. An excretory system has not hitherto been seen. Yolk-glands are absent. An otolith underlies the brain. TheAcoelaare marine.