176 MANUAL OF ZOOLOGY SECT. 



somewhat like a feather, with a main axis and a pair of 

 lateral rows of short slender branches, the pinnules. The 

 arms act as the locomotive organs of the animal, their wav- 

 ing movements propelling it slowly through the water. Tube- 

 feet are not developed as such ; but are represented by a 

 great number of very minute simple processes, the tentacles, 

 which border grooves running along the upper surfaces of 

 the arms and of the pinnules. 



Some of the Crinoidea, the stalked Crinoids (Fig. 99), 

 chiefly occurring at great depths in the sea are supported on 

 a long slender stalk by which they are permanently fixed. 

 In the ordinary feather- stars the larva passes through a 

 stage in which it is attached by means of a stalk like the 

 stalked Crinoids : after a time the stalk becomes absorbed 

 and the young feather-star becomes free. 



Our two species of Antedon live in deep water off the 

 New England coast. 



A remarkable feature of the Echinodermata is the prevail- 

 ing radial arrangement vi their parts, a feature in which they 

 resemble the very much more simply organised Ccelenterata. 

 But underlying this there is to be detected a more obscure 

 arrangement of the body in right and left halves, just as in 

 the bilateral animals we have been more recently dealing 

 with. This deeper bilateral symmetry is almost completely 

 disguised by the radial arrangement of most of the parts. 

 In the larva the symmetry is strongly bilateral and it is only 

 by passing through a remarkable metamorphosis in which 

 parts of the larva are sometimes altogether discarded that 

 the radially constructed adult form is developed. 



