rHE FR TO Ch\':\Z>A TA 



cri/.c 



col'.ectivelv give rise to the cerebral or suprjcesophageal 

 ganglion. The apical plate may thus be denned as a 

 primitive central nervous system a: the apex of the 

 prsoral lobe, being the forerunner and formative centre 

 of the cerebral ganglion of the Invertebrates. 



Although, with the exception of the Crinoids, there is 

 no apic-il plate in the tvpic.-d Echinoderm larva, yet. as 

 noted above, in many cases a curious transiton,- lengthen- 

 ing of the ectodermic cells at the apical pole has been. 



and can be without gxeat 

 ditficulty. observed in lar\"35 

 of star-fishes and sea-urchins. 

 This alone would seem to 

 indicate the former exist- 

 ence of a central nervous 

 svstera at the apex of the 

 pra?oral lobe in the bilateral 

 ancestor of the Echinoderms. 

 The wav in which the 

 primarv blastoccelic cavity 

 of the prajoral lobe can be 

 replaced bv a dilatation of 

 the enteroccel has been de- 

 scribed above, both for Tor- 

 naria and for the larva of AsfL-rijs z:i'^l1!-:s ^Figs. i^i-i::::). 

 In some cases, as in As'^-r:i:j ^irl^^sj. the pra?oral lobe is 

 occupied bv the enterocojl from the verv beginning. In the 

 " Piuteus " larva of the Echinids (sea-urchins^ the pra?oral 

 lobe is much reduced ; but in other Echinoderms, as in 

 the singular l.irva of AsUt:>:.i ;,--/'." :\\c,7. and in the so-called 

 Prachiolaria-larva of the Asterids ^star-fishes") in general, it 

 is verv prominent, and serves as an etfective locomoton 

 [cn-c-fi)!g^ org.tn. 



Fig. 129. — L.ir\M of .-.'-'.'."-.-':.:<-;.' .'.-o, 

 viewed .vs a transp.irent object frora tr.e 

 left side, i^.^tter LUDWIG.I 



<■■;.'..-. Enteric CA\ity. .'..'. Left entero- 

 cce".. communicating with the right entero- 

 co?'. through /.-'. the pn.vorAl lobe. st. 

 Siomodceum. 



