SEA-URCHINS AND SEA-CUCUMBERS. 



805 



and unite, first into T-, and then into H -forms, and 

 then into irregular networks. Meanwhile, fleshy cylindrical 

 columns spring up from the surface, one in each of these 

 interspaces, and presently develop within their substance a 

 similar framework of porous glass ; these soon manifest them- 

 selves to be the spines, and each is seated on a little nucleus 

 of network, on which it possesses the power of rotating. 



At the same time pedicellariae begin to be formed ; and, 

 what is specially marvellous, they are first seen, not on 

 the disk, which alone is to be the future Urchin, but on 

 the interior wall of the helmet, which is even now in pro- 

 cess of being dissipated, and even on the opposite side to 

 that which carries the disk. They commonly appear four 

 in number, arranged in two pairs ; and one can see in them 

 they being, like the suckers, large out of all proportion 

 to the disk the stem, and the three -leaved heads, which 

 already exercise their characteristic snapping movements. 



The disk is meanwhile enlarging its area; and the 

 spines and suckers, gradually lengthening, at length push 

 themselves through the walls of the helmet ; the hanging 

 points and crest of which are 

 fast diminishing by a kind of 

 insensible absorption ; the ci- 

 liary movements become less 

 vigorous, and the mouth closes 

 up. But, correspondently, the 

 Urchin is beginning to acquire 

 its own independent power of 

 locomotion ; for the suckers, 

 now ever sprawling about, are 

 capable of adhering to any 

 foreign body with which they 

 come into contact, and of drag- 

 ging the whole structure about* by their proper contractions. 

 The cilia that cover the thickened fringing band still 

 exercise then: powers, and are the last to disappear. 



YOUNG SEA-URCHIN: 

 DEVELOPMENT OF DISK. 



