448 



HAUIATA : El'HINODERMATA. 



FIG. 678. 



these organs are not fully understood ; but one of their 

 uses is to aid in keeping the spines clean. 



The Echinoids represented by Figs. 673-675 have the 

 mouth below and the vent above, and both central ; and 

 they have the ambulacra 

 in live pairs continuous 

 from the mouth to the 

 vent, and so they are 

 called the Regular Echi- 

 noids. 



Besides these there are 

 many kinds of Sea-ur- 

 chins mostly much flat- 

 tened, and in many cases 

 elongated in one direc- 

 tion more than in the 

 other which have the 

 mouth below, and the 



Finger-Urchin, Rotula Rumphii. Cape Palmas. 



vent not opposite and 



FIG. 679. central, but sometimes be- 



low, and sometimes at one 

 side (Fig. 679), and the am- 

 bulacra not continuous ; 

 and in many cases the shell 

 is very curiously modified, 

 exhibiting holes, fingers, 

 etc. (Figs. 678, 680), quite 

 different from anything 

 seen in the typical Sea- 

 urchins therefore they 



are called Irregular Echi- 

 . -i 



Cake-Urchin, Echinarachnius parma. Gray. 

 North-east coast of the United States. 



