. A text-book of invertebrate morphology. Invertebrates. TYPE PLATTEELMINTHES. 141 tation the cells whicli are to form the mesoderm are separated off from those from which the ectoderm and endoderm are to be derived, so that even before the invagination all the three layers are represented. This, however, is to be regarded as a precocious segregation of the germ-layers, and even within the limits of the few forms whose embrology is known consider- able variations in the time and manner of the differentiation of the mesoderm occur. The result of the invagination is in some cases a solid, bilate
. A text-book of invertebrate morphology. Invertebrates. TYPE PLATTEELMINTHES. 141 tation the cells whicli are to form the mesoderm are separated off from those from which the ectoderm and endoderm are to be derived, so that even before the invagination all the three layers are represented. This, however, is to be regarded as a precocious segregation of the germ-layers, and even within the limits of the few forms whose embrology is known consider- able variations in the time and manner of the differentiation of the mesoderm occur. The result of the invagination is in some cases a solid, bilaterally symmetrical, ciliated embryo consisting of a layer of ectoderm enclosing a central mass of endoderm and mesoderm, in the interior of which a cavity ap- pears surrounded by the endoderm. A depression appears on the ventral surface, which, deepening, finally unites with the enteron and forms the pharyngeal pouch, and gradually the characters of the adult are assumed. In some forms whose ova are provided with comparatively little yolk the embryo leads from an early period a free-swim- ming existence, and in accordance with this a specialized form has been acquired and a slight metamorphosis is neces- sary for the conversion of this larva into the adult condition. In Stylochus the embryo develops into what is known as Ooette's larva, a bilateral ciliated structure with an anterior and posterior tuft of strong sensory hairs, while from the ven- tral surface on either side of the mouth there hang down two ciliated ear-like lobes or lappets. In another form (Thysanozoon) these lappets are much more developed, passing round to the dorsal surface of the body, and their edges are drawn out into four or eight lobes, one of which lies in front of the mouth and another on the dorsal surface, the other two or six lying at the sides of the body and being arranged symmetrically on either side. It seems pig. 75.—Larva of probable that this larva, known as Muller's Thysaaozuon, Mul- kr«
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