zi8 ECHINODERMATA. 



cirri. But this nervous-vascular structure is much too 

 intricate to be described here. 



The water-vascular system consists as usual of a circum- 

 oral ring and radial vessels. These lie under the correspond- 

 ing blood-vascular system. But the system is divergent in 

 several ways ; (a) water passes into it by several ciliated and 

 branched water-tubes which hang from the ring, and from 

 the origins of the radial vessels, into the body-cavity ; (b) 

 water passes from the exterior into the body-cavity by 

 numerous (1500 on the disc of Antedon rosacea) ciliated 

 water-pores which pierce the disc, and sometimes the arms 

 also ; (c) " the radial water-vascular vessels give oif 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 resembling sexual 

 intertwining 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, 

 in males by one or two fine canals. 



There are about 400 living species in twelve genera, but 

 about 1500 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. 



The Crinoids are more nearly related to the extinct 

 Cystoids and Blastoids than to the other classes. 



Classification of Crinoidea. 



1. Palseo-crinoidea (= Tesselata). Palaeozoic forms. The sjfmmetry 



of the calyx is not always pentamerous. 



2. Neo-crinoidea (= Articulata H- iKj/o^w and Marsupiies). 



Mesozoic and recent. The calyx always has pentamerous 

 symmetry. The recent forms include the stalked Pentacrinus, 

 Rhizocrinus, etc., 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. Marmpites is an extinct Crinoid which 

 has no stalk. 



