304 EVENINGS AT THE MICROSCOPE. 



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 temporary nurse, within which the 

 future Urchin 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 inde- 

 pendent life ; when the whole constitution of the nurse 

 will waste away and vanish ! 



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 interior 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 

 Muller 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, forms the pendulum ; and then 

 the newly formed disk represents the face of the clock, 

 only it is put on the side instead of the front. Now this 

 tiny disk gradually grows into the form and assumes all 

 the organs of the Urchin, while the enveloping nurse, flesh, 

 rods, and all, wastes away to nothing. 



The disk, soon after its appearance, is seen to bear pro- 

 minences on its surface, in which is traced the figure of a 

 cinque-foil, the elements being five warts set symmetri- 

 cally. These lengthen and grow into suckers, essentially 

 identical with those of the adult, but most dispropor- 

 tionately large. In the five triangular interspaces between 

 these, little points and needles of solid calcareous glass 

 begin to form, very much like the crystals that shoot 

 across a drying drop of a solution of some salt ; these catch 



