Sea Urchins. 



55 



shaped jaw (fig. 33), which itself consists of two sym- 

 metrical halves. Twenty-five accessory pieces are ap- 

 pended to these parts, and the whole apparatus is moved 

 by thirty muscles (fig. 32). This apparatus is fixed by 



FIG. 33- 



99 



Jaws of the Sea Urchin. A, two jaws seen laterally ; B, interior view 

 of a single jaw ; ft, surface of jaw ; /, teeth. 



muscles and fibrous bands to calcareous loops which 

 project inwards at the mouth end of the shell, and it 

 can easily be dissected in the common sea urchin. 

 The larvx of sea urchins are pluteus-like, containing 

 a calcareous skeleton. 



Two types of sea urchins are found in American 

 seas. One like the common Strongylocentrotus dro- 

 bachiensis is globular or slightly flattened, with am- 

 bulacral areas extending from pole to pole, and with 

 the mouth and anus at opposite poles ; the other type, 

 represented by the heart-urchin (Schizaster fragilis) 

 found in deep water, has the anus not opposite to but 

 approximated to the mouth, and the ambulacral rows 

 not extending from pole to pole, but in petal-like 

 areas on one surface of the shell alone. 



