THE PHYLUM ECHINODERMATA 



Other Holothuroidea. Thyone and Cucumaria, a species with five distinct 

 rows of tube feet, represent a type of sea cucumber in which the body form 

 is not highly speciaHzed by extreme modifications and loss of parts. In 

 contrast, Leptosynapta, a burrowing type, lacks tube feet altogether; its echino- 

 derm characteristics are revealed, however, by the presence of minute skeletal 

 plates and five radial muscle bands, visible through the translucent body wall. 

 One of the more highly modified holothurian types is represented by Psolus 

 chitinoides, a species common on the North Pacific coast. In this form the 

 dorsal surface is protected by scale-like skeletal plates, and the ventral surface 

 resembles the creeping foot of a mollusk, except for the presence of three 

 rows of tube feet. 



The Class Crinoidea 



Members of the class Crinoidea have branched arms and are often attached 

 to the bottom by a stalk, which joins the aboral surface of the disk. This 

 gives the animal a flower-like appearance and is responsible for the common 

 name, "sea lily." Even the type represented by Antedon, the so-called feather 

 star, which has no stalk and is not permanently attached, bears a tuft of 

 aboral filaments by which it commonly clings to the substrate (Fig. 16.15). 

 In all crinoids, the oral surface is uppermost; the mouth occupies the center 

 of the oral surface of the disk, surrounded by five highly branched arms. 

 The anus also lies on the oral surface, near the mouth, at the tip of an anal 

 papilla. Ciliated ambulacral grooves radiate from the mouth, traversing the 



Fig. 16.16. Generalized, sche- 

 matic life cycle of a crinoid, 

 based on Anledon. The free- 

 swimming larva {A) is relatively 

 undifferentiated; it attaches and 

 transforms into a pentacrinoid 

 stage (fi), which later meta- 

 morphoses into a free-swimming 

 feather star (Q. (Adapted from 

 H. B. Fell, 1948, Biological Re- 

 views, vol. 23, printed by per- 

 mission of the Cambridge 

 University Press.) 



507 



