ECHINODERMATA. 



139 



Setting out from this point to trace the gradual developement of 

 organization in the Echinodermata, we shall observe a progressive 

 concentration of their entire structure. The central part, or vis- 

 ceral cavity, so small in the Comatula when compared to the 

 complicated rays derived from it, enlarges in its proportional di- 

 mensions as the viscera contained within it become more perfect 

 in their structure ; whilst, on the other hand, the radiating or po- 

 lyp form, so visible in Encrinus and Comatula, becomes obliterated 

 by degrees, until, at length, almost all vestiges of it are lost, or but 

 obscurely recognisable. 



In the Gorgonocephalus (Jig. 59), the proportionate size of the 

 rays when compared with that of the central disc still preponde- 

 rates very considerably, although even here some concentration is 

 manifest. The secondary articulated filaments appended to the 

 rays of Comatula are F- lgt 59. 



no longer recognis- 

 able, their place be- 

 ing supplied by the 

 continual division 

 and subdivision of 

 the rays themselves ; 

 the same end, how- 

 ever, is obtained in 

 both cases, for the 

 numerous jointed 

 and flexible rays 

 of Gorgonocephalus 

 still form so many 

 legs, enabling the 

 creature to drag it- 

 self along the bottom of the sea, or to entwine itself among the 

 submarine plants, as well as supplying the office of tentacula in 

 securing food. 



Continuing our progress towards more perfect forms of these 

 remarkable animals, we at length arrive at genera in which the 

 rays become divested of all elongated appendages, either in the shape 

 of articulated lateral filaments or dichotomous ramifications. In 

 Ophiurus, for instance (Jig. 60), the rays are long and simple, re- 

 sembling the tails of so many serpents a circumstance from whence 

 the name of the family is derived ; nevertheless, on each side of 

 every ray we still trace moveable lateral spines, which, although 



