xvi ECHINODERMATA 519 



place very gradually as growth proceeds. The mouth becomes open 



B. 



FIG. 391. Oral and aboral views of young sea-urchins immediately after the metamorphosis. 



(After von Ubich.) 



A, oral view of young imago of EaMnus microtiiberculatus. B, aboral view of voting imago of Xti'uii- 

 gylocentrdtita liriiln.t. Hj-Bs, the five basal (genital) plates, uh.x/i, aboral spines of imago ; az.t, azygous 

 (\a imary) tentacle ; ilent, rudiment of tooth ; eji.r, cpiiiruiul veil ; j, rudiment of jaw ; o, position of adult 

 mouth ; o.s/j, oral spines of imago ; i>eil, pedicellaria ; Tj-Ts, the live terminal (ocular) plates ; t.f, pam-d 

 tube t'l-ct. 



by the shrivelling of the epineural veil, and if the little animal be 

 kept in salt water which is aerated by forcing air into it, and in 



