Habits and Structure of Triclads.—The common Planaria (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 in P. punctata (Fig. 16, A), P. mrazekii, and P. 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 Planarian Cotyloplana (D) is the same structure, but the two suckers of Dicotylus (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 the Zostera, and so to resist the loosening action of the waves.
Fig. 16.—Suckers of Triclads. A, Planaria punctata Pall.; a, dorsal surface of head; b, ventral surface (freely moving) showing the sucker; c, sucker contracted (after Hallez): B, ventral surface of head of Dicotylus pulvinar Gr., from Lake Baikal (after Grube): C, dorsal surface of Procotylea fluviatilis Gir. (after Girard): D, sucker of Cotyloplana whiteleggei Sp. (after Spencer): E, ventral view of head of Planaria alpina Dana (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 of Bipalium has 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 a Planaria lactea uses the lobes of its head. Further examination shows that the margin of the head of Bipalium is 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 of Geoplana, and in one or two species of Rhynchodemus.[[53]] Some Land Planarians (a species of Rhynchodemus from Ceylon, and a Dolichoplana from 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) in Bipalium kewense, flagellated structures, bent into a V-form and with a slender thread attached to one end (Shipley). In Geoplana coerulea these 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 anomalous Leimacopsis terricola Schm. from the Andes[[54]] and in Dolichoplana it 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 kewense pursues 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, a Bipalium may 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 young worm.[[55]] Geoplana typhlops, a Tasmanian species, is also blind, and pursues worms, as does G. 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.—In Planaria lactea the 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-called uterus (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).
Fig. 17.—Diagrammatic view of the structure of Planaria (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 above mgr; vd, vasa deferentia; vs, vesicula seminalis; yg, yolk-glands; yo, openings of the yolk-ducts into the oviducts.