12 



ECHINODERMA GENERAL DESCRIPTION 



r.pc 



Fio. X. 



ontogeny of Antedon suggests their division into two groups 

 (Fig. X.) : one formed around the upper, oral coelom (l.pc, 

 i.e. the left posterior coelom of the Di- 

 pleurula), which gradually encircled the 

 oesophagus; the other around the lower, 

 aboral, or apical coelom (r.pc, i.e. the right 

 posterior coelom of the Dipleurula). The 

 P former set were affected by radiate sym- 

 metry before the others, and in Antedon 

 larya are represented by five large plates, 

 the " orals " (0). The latter set form the 

 plates of the aboral side of the adult 

 Echinoderm. In a Pelmatozoan they form 

 the dorsal cup (B and IB) and the ossicles 

 of the stem (col) when that organ is pre- 

 sent. 



We have now traced the history of 

 the Echinoderma up to a form fixed 

 aborally, and with rays, normally five in 

 mber > Proceeding from the mouth and 



hydropore ; fp, fixing plate of underlying the hydrocoel ring or hydro- 

 stem, " dorso central "; for . J _ J . / . 

 other letters, see adjoining text. CITCUS. Ihese ray 8 involved Other 01 the 



internal organs, notably portions of the 



oral and aboral coelom, and accompanying them was a develop- 

 ment of epithelial nerves and a circumoral nerve ring. The 

 dividing wall between the right and left posterior coeloms, the 

 dorsal mesentery of the Dipleurula, now lies horizontally or 

 transverse to the long axis. A new vertical mesentery, both 

 above and below, is formed by the tissue separating the in- 

 curved ends of the oral and apical coeloms respectively. On 

 the inner walls of these coeloms, adjoining this mesentery, is a 

 thickening of the endothelium (ax in Fig. VIII. 5), to form event- 

 ually a strand passing up to the main axis through the coil of the 

 gut, and known as the " axial cord." This, in the adult, originates 

 the gonads, which seem at first to have been expelled through an 

 aperture in the body wall between mouth and anus, as seen in 

 Holothurians and some Cystidea. Subsequently this becomes 

 involved in the radiate symmetry. 



The phylogenetic stage thus reconstructed on the evidence of 

 embryology and palaeontology corresponds on the whole to the 

 stage imagined by Semon (1888), and named by him Pentactcea 

 (five-rayed). The question arises : How far does this represent the 

 ancestor of all Echinoderms ? There can be no doubt that this 

 actually was a stage in the history of the fixed Echinoderms 

 (Pelmatozoa) ; that it was also a stage in that of the free Echino- 

 derms (Eleutherozoa), is coming more and more to be the opinion 



