SEA URCHINS AND SEA-CUCUMBEKS. 



303 



grace and with a dignified deliberation ; the crest being 

 always uppermost, and the perpendicular position invari- 

 ably maintained. It does not appear capable of resting, 

 its movements depending on incessantly vibrating cilia. 

 These organs we perceive densely cloth- 

 ing the long ear-pieces, but more espe- 

 cially accumulated and more vigorous, 

 in a thickened, fleshy band, which 

 passes partly round the whole helmet, 

 at the origin of these pieces. 



You do not discern the slightest re- 

 semblance of form between this little 

 slowly-swimming dome and the spined 

 and boxed Urchin which crawls over 

 the rocks ; and you wonder by what 

 steps the tiny atom of one-fortieth of an 

 inch in length is led to its adult stage. 

 Fortunately I can satisfy your curiosity 

 on this point, not indeed from my own 

 observations, but from those of Professor 

 Johann Miiller, whose discoveries of 

 the developments of these and kindred 

 animals are among the most interesting, because the most 

 startling, of the marvels which modern zoology has re- 

 vealed to us. The whole process is full of surprising 

 details, to which the change of the caterpillar to a chrysalis, 

 and the chrysalis to a butterfly, presents no parallel, won- 

 derful as those changes of form appear and are. There we 

 have but modifications of outward form, produced by the 

 successive moults or castings of the external skin, and the 

 gradual growth of the animal, which has from the first 

 been present, though veiled. But the construction of the 

 Sea-Urchin is by no means a process of skin-casting, nor 

 has it any recognised parallel in the whole economy of 

 natural history. It is a development perfectly unique. I 

 will endeavour to make you acquainted with the results 



LAEVA OF SEA-TTBCHIN'. 



