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«a (Fig. 75), may be traced back to a con- ^^^^ ^a.-va (after 

 dition such as that described in Goette's 

 larva, the two lappets of that form having united in front of 

 the mouth, while their lines of attachment have become more 



