246 ECHINODERMA. 



passes from the exterior into the body cavity by numerous ( 1 500 on the 

 disc of Antedon rosaced] ciliated water pores which pierce the disc, and 

 sometimes the arms also ; (c) " the radial water vascular vessels give off 

 alternately to the right and left, in groups of three each, delicate tubular 

 branches, respiratory in function, which form the tentacles homologous 

 with tube feet." 



The sexes are separate, and a process suggestive of sexual union 

 has been observed in Antedon. The reproductive organs extend as 

 tubular strands from the disc along the arms, but are rarely functional 

 except in the pinnules, from each of which the elements burst out by 

 one duct in females, by one or two fine canals in males. 



There are about 400 living species in twelve genera, but about 1 500 

 species in 200 genera are known from the rocks. The class is obviously 

 decadent. It is represented in the Cambrian, and attained its maximum 

 development in Silurian, Devonian, and Carboniferous times. 



The oval ciliated larva of Antedon^ the only one known, is less quaint 

 than that of other Echinoderms. 



Classification of Crinoidea. 



1. Pakeo-crinoidea ( = Tesselata). Palseozoic forms. The symmetry 



of the calyx is not always pentamerous. 



2. Neo-crinoidea ( = Articulata + Holopus and Marsupites}. 



Mesozoic and recent. The calyx always has pentamerous 

 symmetry. The recent forms include the stalked Pentacrinus, 

 RhizocrimtS) <5rV., and the free Comatulids, which pass through 

 a stalked Pentacrinus stage, e.g., Antedon. 



Holopus is a remarkable deep sea form with direct ancestors 

 in the Upper Silurian. Marsupites is an extinct Crinoid which 

 had no stalk. 



Class BLASTOIDEA. Wholly extinct. 



The Blastoicls are first found in the Upper Silurian, later than Cystoids 

 and Crinoids ; they had their golden age in the Carboniferous and 

 Devonian times, but then disappeared. Their body was ovate, with five 

 ambulacral areas, with each groove of which jointed pinnules were 

 associated. 



Class CYSTOIDEA. Wholly extinct. 



The Cystoids are first found in the Lower Silurian rocks, had their 

 golden age in Upper Silurian times, and died out in the Carboniferous 

 ages. Their body was ovate or globular, sessile or shortly stalked, 

 covered with polygonal plates often irregularly arranged. Some (accord- 

 ing to Bell, the more primitive) types were "never fixed, and had not 

 fixed ancestors." They seem usually to have borne two to five feeble, 

 unbranched arms. 



Both Cystoids and Blastoids seem to have been half smothered in lime, 

 and perhaps this is in part the explanation of their extinction. 



