320 EVENINGS AT THE MICROSCOPE 



To return now to our .little E. miliaris, which has "been 

 all this time coursing round and round his saucer, wonder- 

 ing, perchance, at the narrowness and shallowness of the 

 White Sea in which he finds himself. Again we peer, 

 lens to eye, over the bristling surface, and discern, shoot- 

 ing up amid the spines, and almost as thickly crowded 

 as they, multitudes of the tiny organs which have caused 

 so much doubt and discussion among naturalists. Muller, 

 the great marine zoologist of Denmark, who first discov- 

 ered them, thought them parasitic animals, living pirati- 

 cally upon the unwilling Urchin, and accordingly gave 

 them generic and specific names. The term PcdiceUaria, 

 which he assigned to his supposed genus, is that by which 

 modern naturalists have agreed to call them still, though 

 the word is not now used in a generic sense, since it is 

 indubitably established that they are not independent ani- 

 mals, but essential parts of the Urchin itself. Miiller de- 

 scribed three distinct sorts, and I have added a fourth 

 to the number; they are named P. triphytta, tridens, globi- 

 fera, and stereophylla. They all agree in these particulars 

 that each has a long, slender, cylindrical, fleshy stem, 

 through the centre of which runs an axis or rod of cal- 

 careous substance; that the base of the stem rests on the 

 skin of the Urchin; that on the summit is placed a head 

 consisting of three pieces, which are capable of being 

 widely opened and of being closed together, at least at 

 their tips; that the edges of these pieces, which come into 

 mutual contact, are furnished with teeth, which lock into 

 each other; that the head-pieces (like the stem) consist of 

 calcareous centres, clothed with flesh; that, besides the 

 opening and shutting of the head, the stem can be swayed 

 from side to side; and that all these movements are spon- 



