PHYLUM ECHINODERMATA 



205 



Echinoidea in General. The common sea-urchins just de- 

 scribed live principally on rocky shores. The cake-urchins 

 (Fig. 144) live at or near the surface of the sand; a common 



mp 

 / pod' 



pod 



FIG. 144. Aboral view of a " sand- 

 dollar," Echinarachnius parma. m.p, madre- 

 porite; pod, small tube-foot; pod', flattened 

 respiratory tube-foot. (From the Cam- 

 bridge Natural History.) 



FIG. 145. Aboral. view 

 of the test of a heart- 

 urchin, Brissopsis lyrifera. 

 Af, anus. (From Sedg- 

 wick, after Claus.) 



form on the eastern coast of North America is the sand-dollar, 

 Echinarachnius. The heart-urchins (Fig. 145) bury themselves 

 in the mud to a depth of from a few inches to a foot. 



5. CLASS IV. HOLOTHURIOIDEA. SEA-CUCUMBERS 



Distinctive Characteristics. Elongated on principal axis; 

 body-wall muscular with small calcareous plates; contractile 

 tentacles around mouth; no external madreporite. 



Structural Peculiarities. The most striking external features 

 of the sea-cucumber (Fig. 146) are its muscular body-wall almost 

 devoid of large skeletal plates, its branching tentacles surrounding 

 the mouth, and its lateral position when at rest or moving about 

 on the sea bottom. 



The water-vascular system (Fig. 147) is homologous to those 

 of the other classes of echinoderms. There is a circular canal 

 around the oesophagus (2), five radial canals (i) which end 



