Echinodermata. 



343 



and the cavity of this depression forms the digestive cavity. 

 The mouth is formed later by a new opening made through 

 the outer wall into the first cavity, and the original opening 

 becomes the anus. So far the young sea urchin is very 

 unlike the adult ; but after a time this larva begins to 

 transform into the real sea urchin, and soon the little sea 

 urchins, about the size of pins' heads, are found crawling 

 up the sides of the glass vessels in which they are kept. It 

 should be noted that the larval sea urchin is bilaterally 

 symmetrical, whereas the adult 

 is, apparently at least, radially 

 symmetrical. 



Other Forms of Echinoids. 

 In addition to the common 

 apple-shaped sea urchins, we 

 find very greatly flattened 

 forms, such as the sand cake, 

 or sand dollar, also called cake 

 urchin. The mouth is central, 

 but the anus is at one edge. 

 Slightly different is the key- 

 hole urchin, with narrow open- 

 ings through the shell (not 

 communicating with the body cavity). There are some elon- 

 gated urchins, showing rather marked bilateral symmetry. 



FIG. 192. SAND DOLLAR; CAKE 

 URCHIN. 



The spines are removed from most of 

 the surface. 



CLASS IV. HOLOTHUROIDEA. 



Sea Cucumbers. Our larger holothurians are cucumber- 

 shaped, hence the common name, sea cucumber. The 

 mouth is at one end and the anus at the other. Around 

 the mouth is a 'circle of tentacles, with which food is 

 taken. Along the sides, usually in five distinct rows, are 



