XVI EGHINODEEMATA 519 



place very gradually as growth proceeds. The mouth becomes open 

 ® 



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



(After von tJbich.) 



A, oral view of young imago of Echirms microtuhercidatus. B, aboral view of young imago of Utron- 

 gylocentrotus li/vidus. B1-B5, the five basal (genital) plates, db.sp, aboral spines of imago ; az.t, azygous 

 (primary) tentacle; dent, rudiment of tooth; ep.v, epineiiral veil; j, rudiment of jaw; 0, position of adult 

 mouth ; o.s}), oral spines of imago ; ped, pedicellaria ; Ti-Tg, the five terminal (ocular) plates ; t.f, paired 

 tube feet. 



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 



