202 COLLEGE ZOOLOGY 



4. CLASS III. ECHINOIDEA. SEA-URCHINS 



Distinctive Characteristics. Pentamerous, without arms or 

 free rays; skeleton usually of twenty columns of firmly united 

 plates, five pairs of ambulacral rows, and five pairs of inter- 

 ambulacral rows. 



Structural Peculiarities. The starfish type may be changed 

 to that of the sea-urchin quite easily. The latter (Figs. 141- 



FIG. 141. Aboral surface of a sea-urchin, Strongylocentrotus drobachiensi?. i, ex- 

 panded tube-feet; 2, spines. (From Shipley and MacBride, after Agassiz.) 



142) resembles a starfish whose aboral surface has become 

 exceedingly reduced, being represented by a small area, the 

 periproct (Fig. 142, 2), and the tips of whose arms have at the 

 same time been bent upward and united near the center of 

 the aboral surface. 



The skeleton of the sea-urchin is known as a shell or test, and 

 is shown in detail in Figure 142. The apical system of plates 

 contains the madreporite (3), four other genital plates (4), with 

 genital pores, and five ocular plates (5), each with a mass of pig- 

 mental cells. There are five pairs of columns of ambulacral 



