X 



PODAXONIA 



377 



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

 dermic thickening. This thickening becomes detached from the 

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

 iu Phascolosoma vulgare~) it becomes divided into two to four segments. 

 These may be regarded as ganglia of the nerve cord. In Phasc,oloxuiii 

 gouldii the mesoderm also becomes divided into segments (men 1 , mes 2 , 

 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 



h. 



FIG. 305. A Trochophore larva of P/iavcolosoinn ruli/nre a little niorr tlian thirty-six 

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



'il>, apical plate with its tuft of cilia ; eh, chorion still investing the embryo ; hJi, heafl-blastnna ; 

 mlr, rnetatroch ; ii.tr, prototrocli ; oc, eye-spots; tnni, stomodaeum ; t.li, trunk-blastema. 



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

 ated. Somewhat later the coelomic cavity appears ; in Pkasco/omninr 

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

 together and form one undivided cavity. In P. riili/iire, however, in 

 which the mesoderm is unsegmented, the coelom a]p(iars 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 



