SEA-URCHINS AND SEA-CUCUMBERS 



to a chrysalis, and that of the chrysalis to a butterfly, 

 present no parallel, wonderful as those changes of form 

 appear and are. There we have but modifications of out- 

 ward form, produced by the successive moults or castings 

 of the external skin, and the gradual growth of the ani- 

 mal, 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 recognized parallel 

 in the whole economy of natural history. It is a develop- 

 ment perfectly unique. I will endeavor 

 to make you acquainted with the results 

 arrived at from the researches of the em- 

 inent German zoologist to whom we are 

 indebted for almost all we know on the 

 matter. 



Let me first premise that this beauti- 

 ful helmet- shaped creature is not the future 

 Urchin; and, strange to say, that only a 

 very small portion of the present structure, 

 namely, the stomach and gullet, will enter 

 into its composition. The helmet is a kind 

 of temporary nurse, within which the fu- 

 ture Urchin is to be formed, and by which LARVA OF SEA - URCHIN - 

 it is to be carried from place to place by its ciliary action, 

 while the young animal is gradually acquiring the power 

 of independent life, when the whole constitution of the 

 nurse wastes away and vanishes! 



The first trace of the young Urchin is a filmy circular 

 plate, which is not symmetrical with the helmet, nor 

 formed even on the same plane, but appears obliquely 

 fixed on the exterior of the stomach, on one side, close 

 to the arch of transparent flesh which stretches from one 



