PLUTEUS AND METAMORPHOSIS OF ECHINUS. 231 



vaulted body, resembling epaulettes. It is also pro- 

 vided with a fringe of cilia encircling the dome,, and 

 continued upon all the columns up one side and down 

 the other. 



The first evidence of the approaching change of 

 this Pluteus into an Echinus is the formation of a 

 round disc like a clock-face, with five double lines 

 like pointers radiating from its centre, and at their 

 extremities expanding into circles, having a double 

 outline, which may be soon seen to form the bases of 

 tentacles like those that traverse the ambulacral 

 pores of the adult Echinus, but which are as yet 

 single, not in pairs. The disc or clock-face progress- 

 ively expands, extending over the gelatinous mass to 

 which it owes its origin, and developes tubercles 

 which push through the transparent dome of the 

 Pluteus, and are transformed into spines and ten- 

 tacles that soon manifest their characteristic motions, 

 combining their apparently voluntary actions with 

 the continued vibrations of the ciliated epaulettes of 

 the Pluteus ; so that it can now both swim and creep. 

 Gradually, however, as the growth of the Echinus 

 advances, the Pluteus disappears, and even by the 

 time the disc has expanded half over its surface, 

 little of the latter remains except a few of the calca- 

 reous rods which formed its framework; while the 

 nascent shell of the Echinus, covered now with spines 

 and locomotive tentacles, plainly indicates the ap- 

 proaching completion of this wonderful, and, until re- 

 cently, unheard-of transmutation. 



The attention of naturalists has long been directed 

 to a peculiar animated organization met with upon 



