XVI 



ECHINODEKMATA 



517 



of the basal collar of muscular and nervous tissue so characteristic 

 of the adult Echinoid spine. 



Other calcareous plates, the terminals, corresponding to the 

 ocular plates of the adult, are developed on the left side of the 

 larva outside the amniotic cavity. 



At a time varying between forty-five and sixty days from the 

 time of fertilization, metamorphosis occurs, and the larva changes 



pr.o.a 



la 



p.o.a 



sens 



FIG. 390. Echiuopluteus larva of Echinus esculentus about fifty days old, just about to 



metamorphose, viewed from the left side. (Original.) 



Letters as in Figs. 3SO and 387. In addition, az.t, azygous tentacle, primary tube feet; dent.s, dental 

 sac ; ep.v, epineural veil, covering the position of the adult mouth ; muse, muscles of adult spines ; n.r, 

 adult nerve ring ; sens, sense-organ at apex of primary tube feet ; t.f, rudiments of first pair of tube feet. 



into a young urchin which we may term an imago. The roof of 

 the amniotic cavity becomes absorbed in the centre, and the primary 

 azygous tube feet are protruded. In the centre of the disc of each 

 there is a sense-organ consisting of a conical prominence of elongated 

 ectoderm cells. At the base of each a pair of buds may be detected 

 (t.f, Fig. 390), which are the rudiments of the first pair of tube feet. 

 The peripheral portions of the amniotic roof shrink back, so that 

 the terminal plates which they contain come to lie externally to 

 the tube feet. 



