ECHINODERMATA 



441 



execute tactile movements. At this time the larval skeleton 

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

 as a result of this (Fig 215). The body thereby assumes 

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

 The circumference of the disc has increased more and more, 

 and correspondingly the opening of the invagination has 

 also become enlarged. The amnion-like envelope meantime 

 gradually diminishes in prominence ; at length it forms only 

 a circular fold, surrounding the circumference of the disc, 



Fig. 214.— Plutens larva of Arhacia pitsfiilosa (after Joh. Mullek), The steletal 

 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 are 

 already employed as locomotor organs. In Fig. 216 the 

 young sea-urchin spines are already seen making their 



