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. 



Jaws of the Sea Urchin, A, two jaws seen laterally ; B, exterior view of a 

 single jaw ; /$, rough surface of jaw ; t, teeth ; , radius ; k 1 , lateral piece 

 of alveolus ;/, edge of jaw ; z, lotula. The epiphysis is shown below /. 



muscles and fibrous bands to calcareous loops which 

 project inwards at the mouth end of the shell, and :t 

 can easily be dissected in the common sea urchin. 

 The larvse of sea urchins are pluteus-like, containing 

 a calcareous skeleton. 



Two types of sea urchins are found in British seas. 

 One, like the common Echinus esculmtus, is globular 

 or slightly flattened, with ambulacral areas extending 

 from pole to pole, and with the mouth and anus at op- 

 posite poles ; the other type, exemplified by the heart- 

 urchin (Echinocardium cordatum)iowsM& on sandbanks, 

 has the anus not opposite but approximated to the 

 mouth, which in our native forms is toothless, and the 

 ambulacral rows not extending from pole to pole, but 

 in petal-like areas on one surface of the shell alone. 



