374 



THE ANIMAL KINGDOM 



Madreporit 



Anu5 



Storie ca.nal 

 Esophagus 

 SiphoiT 



Bo(iy xv7aJl 



Gonad. 



l-n-tdsbinz 



Bociy cavity 

 Stomadh. 



Aristotle.^ lan-bern. 



Me-mbrane 



around mouth -^ Teeth 



Figure 19.13. A sea urchin, Arbacia punctulata, with one side of the body wall re- 

 moved and only some of the structures shown. The teeth protrude from Aristotle's 

 lantern, of which only the outer structures are indicated. The digestive tract circles twice 

 around the body, once in each direction. A second tube, the siphon, by-passes the esoph- 

 agus and stomach. Each of the five gonads opens above near the anus. 



l^-)s»ss*!SJisssjsssisssss^sssg3fcsiS!!^^ 



ia? ff^ 



Figure 19.14. Ventral and lateral views of heart urchins {A, B) and sand dollars 

 (C, D), showing modifications for burrowing and for creeping through sand. (After 

 Hyman.) 



Some urchins live on coral reefs where waves are continually break- 

 ing over them. Their spines are as thick as pencils and are used as props 

 to hold the urchins tightly in shallow crevices. They remain for long 

 periods in one place and often carve out a depression in which they sit, 

 feeding upon the debris brought to them by the waves. 



Some urchins are ovoid and have lost much of their radial sym- 

 metry. These, known as heart urchins (Fig. 19.14), plow through the 

 sand just beneath the surface. Ciliated grooves along the ambulacral 

 areas collect fine debris which is eaten, and the long upper tube feet 

 project above the sand for respiration. 



A group of much-flattened echinoids are the sand dollars (Fig. 

 19.14). These creep almost entirely by the action of numerous short 

 spines. They usually move slowly just under the surface of the sand, and 

 use the upper spines to keep a thin layer of sand moving over the top 

 as they creep on their lower spines. The sand dollars also have ciliated 

 grooves that collect fine debris for food. 



