METAZOA ECHINODERMA. 149 



of the arms were covered and led to the mouth which was 

 also covered over by a series of plates. The anus is in 

 the body plates and outside of the arms. The possession 

 of a complete digestive system ending in an anus and 

 entirely separate from the body cavity is a distinctive 

 feature of the Echinoderma, separating this subkingdom 

 from the Coelentera. The body plates in Caryocrinus 

 were pierced by holes which were the openings of the 

 tubes that ran along the inner side of the plates, and which 

 connected with the respiratory organs or hydrospires. 



In the Cystoidea the ambulacra constitute the feeding 

 and not the locomotive system, so that, were we consider- 

 ing function rather than homology, food grooves would be 

 a good name for ambulacra and tentacles an appropriate 

 name for the organs which came out of the openings of 

 the food grooves. It is probable, as we have already said, 

 that the first forms were without food grooves or ambula- 

 cra ; these may have appeared later as hollows scooped 

 out of the surface of the body, so that the ambulacral 

 plates were set in between the body plates. The next step 

 might be to set these on top of the body plates, as in Meso- 

 cystis. Finally, they might be pushed upward still more, 

 until they were off of the body altogether, as in Caryocri- 

 nus, forming the ambulacra of the arms. 



BLASTOIDEA. 



The Blastoids probably sprang from the Cystoids. One 

 of the most generalized Blastoids is Codaster (PI. 260). 

 Here the body is attached by a stem and it is made of reg- 

 ular plates consisting of basals, forked radials, and inter- 

 radials. Its ventral side (PI. 260) is wide and nearly flat, 

 and on this flattened area the five imperforate ambulacra 

 (PL 260, am} are spread out. The many slits (PI. 260, s) 

 of the hydrospires are exposed between the ambulacral 

 areas. The anus (PI. 260, a) is flush with the surface. 



