I 



ECHINODERMATA 



441 



xeciite tactile movements. At this time the larval skeleton 

 begins to break up, and the arms of the Pluteus degenerate 



I^Has a result of this (Fig 215). The body thereby assumes 

 HRiearly the form of a hemisphere with the disc as the base. 

 The circumference of the disc has increased more and more, 

 nd correspondingly the opening of the invagination has 

 Iso become enlarged. The amnion-like envelope meantime 

 adually diminishes in prominence ; at length it forms only 

 circular fold, surrounding the circumference of the disc, 



Fig. 214.— Pluteus larva of Arhacia pustulosa (after Joh. Mullee). The skeletal 

 rods are dark. P, pedicellaria. 



and finally disappears. Thus the amnion seems to become 

 directly converted into the skin of the sea-urchin, and, in 

 fact, would seem to supply that part of the skin which unites 

 the sole-like ventral surface with the arched back. Fig. 

 215 represents a young sea-urchin which still possesses, in 

 addition to the feet, some of the Pluteus arms. Its feet ai-e 

 already employed as locomotor organs. In Fig. 216 the 

 young sea-urchin spines are already seen making their 



