2/0 THE PROTOCHORDATA. 



collectively give rise to the cerebral or supraoesophageal 

 ganglion. The apical plate may thus be defined as a 

 primitive central nervous system at the apex of the 

 praeoral lobe, being the forerunner and formative centre 

 of the cerebral ganglion of the Invertebrates. 



Although, with the exception of the Crinoids, there is 

 no apical plate in the typical Echinoderm larva, yet, as 

 noted above, in many cases a curious transitory lengthen- 

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



and can be without great 

 difficulty, observed in larvae 

 of star-fishes and sea-urchins. 

 This alone would seem to 

 indicate the former exist- 

 ence of a central nervous 

 system at the apex of the 

 prasoral lobe in the bilateral 



'*- \ x ^y/ / / ancestor of the Echinoderms. 



The way in which the 

 Fig. 129. Larva of Asterina gibbosa, primary blastocoelic cavity 



viewed as a transparent object from the 



leftside. (After LUDWIG.) of the prseoral lobe can be 



ent.c. Enteric cavity U Left entero- replaced b dilatation of 



coel, communicating with the right entero- J 



ccei through /./, the prasoral lobe. si. the enterocoel has been de- 



Stomodceum. 



scribed above, both for Tor- 



naria and for the larva of Astcrias vnlgaris (Figs. 121-122). 

 In some cases, as in Asterina gibbosa, the praeoral lobe is 

 occupied by the enterocoel from the very beginning. In the 

 "Pluteus" larva of the Echinids (sea-urchins) the praeoral 

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

 the singular larva of Asterina gibbosa, and in the so-called 

 Brachiolaria-larva of the Asterids (star-fishes) in general, it 

 is very prominent, and serves as an effective locomoton 

 (creeping] organ. 



