In the latter series the primitive body-form was most nearly 

 retained, but all the systems became considerably differen- 

 tiated. 



In the ancestral Holothuroidea the shape became elon- 

 gated antero-posteriorly, and, as a result, the pentagonal 

 arrangement of many of the parts was masked. The calcar- 

 eous deposits in the integument remained as scattered 

 spicules, while strong bands of muscle were developed to 

 strengthen the body wall. At the anterior end, around the 

 mouth, a part of the water-vascular system was prolonged 

 outwards to form a series of large tentacles, while at the 

 posterior end of the body a pair of large branched tubular 

 organs (the " respiratory trees ") in all probability place the 

 coelom in communication with the exterior through the 

 cloaca, and act as nephridia. From such an ancestral form 

 the various groups of Holothuroidea may be readily derived. 

 Some of them (e.c/., Synapta) are probably slightly degen- 

 erate, the respiratory trees and some parts of the water- 

 vascular system being absent. 



In the ancestral Echinoidea the body became more 

 globular, and the calcareous deposits in the integument 

 were increased greatly, so as to form eventually a continuous 

 shell composed of regularly arranged plates, and bearing 

 spines and other calcareous projections on its outer surface. 

 Some of the existing groups of Echinoids have become 

 highly differentiated, but all the systems may be traced back 

 to those of the ancestral form. The water-vascular system 

 has become an important and complicated series of organs, 

 and a very elaborate calcareous apparatus, bearing teeth 

 (Aristotle's lantern), has been evolved in connection with the 

 mouth. 



Returning now to the main branch of the Echinodermata, 

 and tracing it onwards towards the Asteroidea, Ophiuroidea 

 and Crinoidea, we lind that in the common ancestors of 



