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 {29}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. [Illustration: 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; _mg_{1}_, unpaired gut branch over the brain; _mo_, "mouth" of larva; _n_, _n_{1}_, 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; _sl_{1}_, _sl_{2}_, _sl_{3}_, lateral ciliated lobes of the right side; _vl_, ventral ciliated lobe.]