348 EVENINGS AT THE MICEOSCOPE. 



appear and are. There we have but modifications of 

 outward form, produced by the successive moults or 

 castings of the external skin, and the gi-adual growth 

 of the animal, which has from the first been present, 

 thousrh 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 arrived at from the researches of the eminent 

 German zoologist to whom we are indebted for almost 

 all we know on the matter. 



Let me first premise that this beautiful 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 tem- 

 porary nurse, within which the future TJrchin is to be 

 formed, and by which 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 cir- 

 cular plate, which is not symmetrical with the helmet, 

 nor formed even on the same plane, but appears ob- 

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

 close to the arch of transparent flesh which stretches 

 from one of the points of the vizor to one of the ear- 

 points. Herr Miiller compares the larva (which is not 

 helmet-shaped in every species) to a clock-case, of 

 which the vizor, with its hanging gullet and mouth, 

 form the pendulum, and then the newly-formed disk 



