[Illustration: 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 {30}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.