Fig. 11.—Double eye from the cerebral group of Pseudoceros maximus. (After Lang.)
All known Polyclads are hermaphrodite. The male organs, scattered, like the testes of Leptoplana, 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 in Leptoplana (Figs. 2 and 5, ♂ and ♀). In Trigonoporus, a genus once found at Naples, a secondary female aperture has been discovered leading into the female genital canal[[35]]; and in Anonymus, Polypostia, and Thysanozoon (Fig. 7, E, ♂) two or more male pores and penes have been found. Anonymus has 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 that Thysanozoon uses 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 that Prostheceraeus albocinctus and Cryptocelis alba in 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 in Stylochus neapolitanus it 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 alba lays 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 whereas Stylochus pilidium passes through a modified or, according to some authors, a primitive larval stage, its near ally, S. neapolitanus, develops directly. Most Acotylea 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.
Fig. 12.—Section through Müller's larva of Thysanozoon 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. 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 stage D is reached. By the opening outwards of the pharyngeal sheath (ph.sh) the two apertures gm, or true mouth, and m, 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 the rhabdites (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.) the Maricola, the Triclads of the sea; and (iii.) Terricola or 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.