METAZOA ECHINODERMA. 151 



openings thus formed ten are spiracles and one is the 

 anus. 



We have seen that the hydrospiral or respiratory 

 system is not found in the primitive forms of Cystoids. 

 Where it first occurs, it is independent of the feeding 

 system and is on the surface, as in Echinosphaerites. 

 Later, among the Blastoids especially, it is sunk under 

 and crowded closely against the feeding system. 



Various interesting modifications take place in the 

 structure of Blastoids. The body may become elongated 

 and the ambulacra narrow, as seen in Tricoelocrinus obli- 

 quatus Worthen (No. 263), where the lancet plate is cov- 

 ered by the side plates. 



In Nucleocrinus vernueilli Troost (No. 264), the basal 

 plates are very small and sunken; the radials are also 

 reduced in size, while the broad interradials and the nar- 

 row ambulacra make up most of the body. The latter 

 extend from the top or ventral side downward to the 

 lower side. 



One genus of Blastoids, Eleutherocrinus (E. cassedayi 

 Shum., No. 265), shows a peculiar specialization of 

 structure. One ambulacrum has become modified and 

 is found at the top, leaving only four long ambulacra. It 

 is interesting to note that this specialization of the ambu- 

 lacra appears in a stemless and, therefore, a reduced blas- 

 toid. 



CRINOIDEA. 



The Crinoidea may be divided into two series, each 

 one of which begins with stemmed species and ends in a 

 stemless form. 



The more generalized series is represented in the 

 Collection by Haplocrinus, Cupressocrinus, Cyathocrinus, 

 Encrinus, and Marsupites. 



Haplocrinus (No. 266) has the body made up of two 

 circles of plates ; the circle at the base of the body and 



