282 EVENINGS AT THE MICROSCOPE. 



spond with those of the nipple. It is indeed a true " ball 

 and socket " joint, like that of the human hip or shoulder, 

 and is surrounded by a capsular ligament to keep it in 

 place, the muscles which sway the spine from side to side 

 and cause it to rotate being inserted outside the capsule or 

 case. Professor Edward Forbes calculates that upon a 

 large Echinus, such as this dried specimen of E. sphaira, 

 there are more than four thousand spines, every one of 

 which has the structure, the mechanism, and the move- 

 ments that we have been examining. Well may he say, 

 that " truly the skill of the Great Architect of Nature is 

 not less displayed in the construction of a Sea-urchin than' 

 in the building up of a world ! " , 



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, shdbting up 

 amidst 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 discovered them, 

 thought them parasitic animals, living piratically upon the 

 unwilling Urchin, and accordingly gave them generic and 

 specific names. The term pedicellaria, which he assigned 

 to his supposed genus, is that by which modern natu- 

 ralists 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 animals, but 

 essential parts of the Urchin itself. Muller described 

 three distinct sorts, and I have added a fourth to the 

 number ; they are named P. triphylla, tridens, gloUfera, 

 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 calca- 

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



