cliv Introduction. 



these branches are lodged in a separate canal^ leading from the 

 central demi-canal, in which the nerve-cord and the radial water- 

 vessel are both lodged^ outwards. The madreporic tubercle, instead 

 of being a prominent object on the dorsal surface as in Asteriae, 

 is situated on the ventral surface, where it is either distinct but 

 very small as in AstropJiyton, or partially fused with, and concealed 

 by one of the oral plates, so as to communicate only by a very 

 small pore with the exterior. There are no pedicellariae in the 

 Oplimridae. The Ophiuridae have a digestive system consisting of 

 a simple sac without diverticula or anus ; whilst, except in the three 

 genera Astropecten, Ctenodiscus, and Luidia, the digestive tract 

 of the Asteriae is always proctuchous, and is without any excep- 

 tion even in the case of the genus Brisinga, which is described 

 ais being intermediate in character between the two Sub-classes, 

 is prolonged for a greater or less distance into the arms. 



The feet of the Ophiuridae differ from those of the Asteriae, with 

 the exception of the three genera just mentioned, in not possessing 

 terminal suckers. In some further j^oints in which the Ophiuridae 

 differ from the Asteriae they appear to resemble the Echinoidea. 

 Their feet sometimes show a tendency to effloresce into lateral 

 ampullae, and thus to approximate in character to the ambulacral 

 gills and tactile feet of certain Echinoidea ; they possess a calcified 

 peristomial apparatus which finds its homologue in parts of the 

 peculiar manducatory apparatus found in that class, but which is 

 not represented in Asteriae ; and their larvae, finally, are plutei- 

 form, whilst those of the Asteriae are vermiform, and never possess 

 a pseud-embryonic calcareous skeleton. It is mainly upon the 

 descriptions of such transitional or inter-connecting living forms 

 as Brisinga endecaencmos ; and of such fossil forms as Protaster 

 and Paleodiscus, that a justification of the placing the Asteriae 

 and Ophiuridae together in one Class must rest. 



Class, Crinoidea. 



Echinodermata with arms radially appended to a central disc or 

 ' calyx,' which disc at one period of their lives, or permanently, is 

 attached by a segmented peduncle to marine objects. They pos- 

 sess a well-developed dermal skeleton, which is movably articu- 

 lated in the arms, but immovably in the region of the disc. The 

 water- vascular system gives off a great number of tentacular tubules 



