SEA-URCHINS AND SEA-CUCUMBERS 345 



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 ciliary movements become less 

 vigorous, and the mouth closes up. 

 But, correspondently, the Urchin is 

 beginning to acquire its own inde- 

 pendent 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 dragging 

 the whole structure about, by their 

 proper contractions. The cilia that (Development of Disk). 

 cover the thickened fringing band still exercise their 

 powers, and are the last to disappear. 



When the disk has grown to such an extent as to 

 spread over about half of the larval stomach, very little 

 remains of the helmet, except the middle portions of the 

 glassy rods and the ciliary bands; all the rest of this ex- 

 quisitely modelled framework having vanished by insen- 

 sible degrees, no one knows how or where. The stomach 

 and gullet, indeed, are gradually sucked into the ever- 

 growing disk; but all the rest, flesh and rods, fringes, 

 bands, and cilia, waste away to nothing. 



The mouth of the larva has no connection with the 



YOUNG SEA-URCHIN 



