PODAXONIA 



377 



of the ventral nerve cord makes its appearance as an unpaired ecto- 

 dermic thickening. This thickening becomes detached from the 

 ectoderm and sinks inwards, and in Phascolosoma gouldii (but not 

 in Phascolosoma vulgare} it becomes divided into two to four segments. 

 These may be regarded as ganglia of the nerve cord. In Phascolosoma 

 gouldii the mesoderm also becomes divided into segments (mes 1 , mes z , 

 mes 3 , Fig. 306). The anus is formed, about the forty-fifth hour, by a 

 narrow cone of endoderm cells growing out dorsally and becoming 



mlr 



ch. 



FIG. 305. A Trochophore larva of Phascolosoma vulgare a little more than thirty-six 

 hours old. (After two figures by Gerould combined.) 



ap, apical plate with its tuft of cilia ; eh, chorion still investing the embryo ; h.b, head-blastema ; 

 mtr, metatroch ; p.tr, prototroch ; oc, eye-spots ; stom, stomodaeum ; t.b, trunk-blastema. 



attached to a cluster of ectoderm cells which become slightly invagin- 

 ated. Somewhat later the coelomic cavity appears ; in Phascolosoma 

 gouldii spaces appear in each of the inesodermal segments, which fuse 

 together and form one undivided cavity. In P. vulgare, however, in 

 which the mesoderm is unsegmented, the coelom appears from the 

 beginning as an undivided space. 



When the Trochophore sinks to the bottom the prototroch is got 

 rid of by a most peculiar process. The inner ends of the large cells 

 of which it is constituted break down into yolky granules; these 



