190 



COLLEGE ZOOLOGY 



calcareous plates bearing movable spines. Examples: Cidaris, 

 Arbacia, Toxopncustes, Strongylocentrotus, — Sea-urchins; Echin- 

 arachnius. — Sand-dollar; Spatangus. — Heart-urchin. 



Class IV. Holothurioidea (Gr. Jwlos, whole; thourios, 

 rushing) (Fig. 146). Long ovoid; muscular body-wall ; tentacles 

 around mouth. Examples: Holothuria, Thyone, Caudina. — 

 Sea-cucumbers. 



Class V. Crinoidea (Gr. krinon, a lily; eidos, form) (Fig. 

 1 48). Arms generally branched and with pinnules; aboral 

 pole sometimes with cirri but usually with stalk, for temporary 

 or permanent attachment. Examples: Antedon. — Feather- 

 star; Pentacrinus. — Sea-lily. 



1. Anatomy and Physiology of the Starfish — Asterias 



External Features. — The starfishes are common along many 

 sea-coasts, where they may be found usually upon the rocks with 



the mouth down. 

 The upper surface 

 is therefore aboral 

 or abactinal. On 

 the aboral surface 

 (Fig. 131) are (1) 

 many spines (Fig. 

 !33j 8) of various 

 sizes, (2) pedicel- 

 larioe (Fig. 133, 10) 

 at the base of the 

 spines, (3) a madre- 

 porite (Fig. 131, 

 mad), which is the 

 entrance to the 

 water-vascular 

 system, and (4) 

 the anal opening 

 {anus). A glance 



Fig. 131. — The starfish, Asterias rubens, seen from 

 the aboral surface, mad, madreporite. (From the 

 Cambridge Natural History.) 



