ECHINODERMATA 149 



mouth and anus is approximately the same in both. The hydro- 

 spires of blastoids are likewise correlated with the pores beneath 

 the arm bases of certain crinoids, such as Batocrinus, 



The Blastoidea and Crinoidea have probably descended from 

 the Cystoidea, while if a common ancestor for all the Echino- 

 dermata, both fixed and free, be sought, it would possibly be 

 found in the primitive cystoids, the Edrioasteroidea (see p. 157). 



Derivation of name. — Greek echinos, hedge-hog, + derma, 

 skin, in allusion to the spines possessed by many of the members 

 of this phylum. 



The Echinodermata are divided into the following seven 

 classes : — 



Page 



A. Cystoidea iS4 



B. Blastoidea 157 



C. Crinoidea 159 



D. Asteroidea 163 



E. Ophiuroidea 165 



F. Echinoidea 165 



G. Holothurioidea 171 



Type of the phylum Echinodermata, Asterias forbesi, the 

 starfish (Fig. 58). 



Asterias forbesi ranges from Maine to the Gulf of Mexico, 

 but is very common only south of Cape Cod, living from high 

 tide line to a depth of one hundred feet. In summer and autumn 

 it lives in rocky places in shallow water, but seeks greater 

 depths in winter. 



The starfish is a free-moving animal, consisting of five rays or 

 arms united broadly to a central portion, the disk. The mouth 

 is situated in the center of the under or ventral side of this disk 

 and from it radiate five grooves, one in the middle of each arm. 

 These grooves are called the ambulacral grooves, or ambulacra. 



Into each of the ambulacral grooves project four rows of soft 

 tubular bodies with sucker-like extremities. These are the tube- 

 feet, the locomotive organs of the animal, and they form a part 



