DEVELOPMENT OF ECHINODERMS 



3283 



DEVELOPMENT OF A SEA URCHIN. 



(Stages 1-8.) 



armed green snake star ( Ophiactis) , shown in the 

 annexed illustration, being thought to represent 

 the result of such a process. The specimen A 

 consists of two almost similar halves; but the 

 three arms toward the bottom of the illustra- 

 tion, marked d, are smaller than the others marked 

 a, and indicate that this half is the later grown. 

 The specimen B, which is seen from the back, has 

 only just separated itself from its other half. 

 The separation appears to take place by a forci- 

 ble though spontaneous rent, and the edges of 

 the wound subsequently grow together, and not 

 merely heal up but reproduce the lost parts of the 

 animal. As a rule, however, echinoderms repro- 

 duce by the ordinary sexual methods; although 

 this, too, presents peculiarities. Just as a but- 

 terfly does not develop directly from the egg, but 

 passes through the intermediate larval stage of 

 the caterpillar, out of whose chrysalis the butter- 



DEVEI.OPMENT OF A SEA URCHIN. 



(Stage 9. ) 



