NO. II ECHINODERMS AS ABERRANT ARTHROPODS CLARK 1 5 



lie on the aboral (originally anterior) side of the gut and the preoral 

 lobe of the larva becomes enclosed by the rows of skeletal elements 

 (apical plates) which are developed outside the right posterior 

 coelome in all echinoderms except holothurians ; these plates are first 

 laid down in a horse-shoe shaped ring which later closes, as does 

 the hydroccele, to form a complete ring. 



In the asteroids the closure of this curved row of plates is effected 

 far from the point of origin of the preoral lobe on the right or right 

 dorsal side of the larval body ; in the crinoids it is efifected at the 

 anterior end of the larval (posterior pole of the adult) body and 

 encloses the preoral lobe just as the hydroccele does in asteroids. 



The larvae of both crinoids and asteroids attach themselves by 

 the preoral lobe ; but whereas in the crinoids the preoral lobe is quite 

 free of the circumoral vessel and arises from the apical or aboral 

 surface of the adult, in the asteroids the preoral lobe is encircled 

 by the water vascular ring and its withered vestige springs from the 

 oral surface of the adult disc. 



In the asteroids the mouth has shifted from the ventral surface onto 

 the left side of the body, but has gone no further. Since the preoral 

 lobe disappears as an appendage from the oral surface within the 

 hydroccele ring it is evident that the apical nervous system of the 

 crinoids, which appears to be intimately connected with the preoral 

 lobe, must be represented by a ventral nervous system in the 

 asteroids. 



In the crinoids the circumoral nerve ring and its extensions be- 

 neath the ciliated ambulacral grooves of the disc, arms and pinnules 

 is associated only with the latter and with the ventral surface of the 

 tentacles. In the asteroids the similarly situated ectodermal ner- 

 vous tracts are in connection with a diffuse ectoneural plexus found 

 throughout the ectoderm and at the mouth with an endoneural plexus 

 which is the central portion of the so-called endodermal nervous 

 system. The deep oral nervous system, consisting of a double cord 

 in each radius just within the radial nerve thickening of the ectoneu- 

 ral system and centering in a more or less complete ring about the 

 mouth, said to be exclusively motor in function, possibly corresponds 

 to the deep oral system of the crinoids which consists of paired cords, 

 one on either side of the water tube ; but the latter is connected with 

 the apical nervous system and the former is not. The apical nervous 

 system of the asteroids, motor in function, consists of a cord in the 

 mid-radial line of each arm and appears to develop from the dorsal 

 peritoneum with which it remains in continuity. It dififers from 

 that of the crinoids in being radial (intersomatic) instead of inter- 



