EMBRYOLOGY OF ECHINODERMS. 



135 



Ophiuraris. 



Pig. 183 represents an Ophiuran undergoing the same process 

 of growth, at a period when the larva is most fully developed, and 

 before it begins to fail. By the limestone rods which support the 

 arms, the Pluteus of the Ophiuran, here represented, resembles 

 that of the Sea-urchin more than that of the Star-fish, while by 

 the character of the water-tubes and by its internal organization 

 it is more closely allied to the latter. It differs from both, how- 

 ever, in the immense length of two of the arms ; these arms 

 being the last signs of its plutean condition to disappear ; when 

 the young Ophiuran has absorbed almost the whole Pluteus, it 

 still goes wandering about with these two immense appendages, 

 which finally share the fate of all the rest. Fig. 182 represents 



Fig. 182. 



an Ophiuran at the moment when the process of resorption is 

 nearly completed, though the arms of the Pluteus, greatly di- 

 minished, are still to be seen protruding from the surface of the 

 animal. 



This mode of development, though common to all Echmo- 



Fig. 182. Ophiuran which ha3 resorbed the whole larva e.xcept the two long arms, y y' Umestooe 

 rods of youug Ophiuran, r miildle of back ; lettering as in Fik. 183. 



