ECHINODERMA GENERAL DESCRIPTION \ i 



on the aboral side of the right posterior coelom ; indeed, the mouth 

 itself is not at the future oral pole, for the larval mouth closed 

 early, and the place where it was became arched over by lips 

 of ectoderm, which formed a " vestibule " (v). This vestibule it 

 is that gradually moves up ; a fresh mouth ultimately breaks 

 through into it, and the lips again unfold at the new oral pole. 

 The connection of the anterior coelom, through the parietal canal 

 and the hydropore, with the exterior, persists (cf. Fig. VIII. 1 and 

 3) ; the hydrocoel opens into the parietal canal by the stone 

 canal at a later period (Fig. VIII. 5). 



There is reason to believe that some of the early Cystidea 

 (Amphoridea, p. 43) had an internal structure scarcely more 

 advanced than Fig. VII. But the fixed stage had further effects. 

 The most notable was the prolongation of ciliated and tentacu- 

 liferous grooves from the mouth, accompanied by processes from 

 the hydrocoel. At first there were three such radial extensions : 

 anterior, right, and left, since the 

 presence of anus and hydropore, and 

 the absence of hydrocoel on the posterior 

 side prevented extensions in that direc- 

 tion (Fig. IX.). The five rays, so char- 

 acteristic of Echinoderma, were produced 

 by the forking of the right and left 

 rays. It was only at a later date, when 

 the hydrocoel had grown into a ring 

 round the oesophagus, that the five rays 

 could proceed equally from this ring. Fir IX 



The division of the rays into a pair The pentame '; ism ' of Echino . 

 enclosing anus and madreporite, and ( ierma contrasted with a regular 



, , , . . -i-ii pentamerism. 0. mouth ; As, anus ; 



known as the bivmm, and the three between them is the madreporite. 



others (anterior, right ant. and left fe^tawy pL^ ^tSSto," 



ant.), known as the trimum, is Opposed which 1 and 5 are thebivimn, 2, 

 /'. ^ r 3, 4, the tnnum. i, 11, in, iv, v are 



to this fundamental Structure. It must the live rays of a Pelmatozoan. 



further be noted that this bilateral 



symmetry of the rays has nothing to do with the bilateral nature 



of the Dipleurula. 



While these changes were in progress the formation of stereom 

 continued. At first there were only spicules deposited in the 

 mesenchyme (see Fig. II. 1, and Fig. III.). These enlarged and 

 fused into plates, which eventually became so large as to abut 

 on one another. These plates were arranged in the mesoderm 

 beneath the ectoderm. An account of their arrangement and 

 structure will be found under Amphoridea (p. 45). Through 

 the fixation below and the radiation of the hydrocoel and food- 

 grooves above, these plates gradually came to lie in definite 

 positions and to assume a definite number, shape, and size. The 



