xvi ECHINODERMATA 519 



place very gradually as growth proceeds. The mouth becomes open 



asp 



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



(After von (Jbich.) 



A, oral view of young imago of Echinus microtuberculatus. B, aboral view of young imago of Utron- 

 gylocentrotus lividus. Bj-Bs, the five basal (genital) plates, db.sp, aboral spines of imago ; az.t, azygous 

 (primary) tentacle ; dent, rudiment of tooth ; ep.r, epineural veil ; j, rudiment of jaw ; o, position of adult 

 mouth ; o.sp, oral spines of imago ; ped, pedicellaria ; Tj-Ts, the live 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 



