I. ASTEROIDEA 



295 



form the roomy body cavity of the adult, the membranes separating them furnish- 

 ing the mesenteries. The right hydroccele remains rudimentary; the left, which 

 has the external opening, separates into (i) a smaller anterior portion, the anlage 

 of the ampulla and axial sinus; (2) the connecting duct or stone canal; and (3) 

 a posterior cavity, the hydroccele in the narrower sense. The latter surrounds 

 the oesophagus (ring canal) and sends off five radial diverticula, the anlagen 

 of the radial canals, which form the basis of the conversion of the bilateral 

 larva into the radially symmetrical adult (fig. 284). It is a question as to 

 which group of Echinoderma is the most primitive, but ease of treatment 

 makes it best to begin with the Asteroidea. 



FIG. 283. FIG. 284. 



FIG. 283. Three stages in the development of the coelom and \vater-vascular system 

 (after Bury and McBride). a, ampulla; b, stone canal; c 1 , c~, left and right coelom 

 sacs; d, hind gut with anus; h l . Ir, left and right (rudimentary) hydroccele sac; he, 

 common anlage of hydrocoele and coelom; m, stomach; s, stomocleum and mouth. 



FIG. 284. Formation of Ophiuran from the pluteus larva uifter Miiller, from 

 Korshelt-Heider) . 



Class I. Asteroidea (Starfish). 



Two parts can be recognized in the body of a starfish, a central disc and 

 the arms, usually five in number, which radiate from it (fig. 290). The 

 relations in which these stand to each other vary between two extremes. 

 In many starfish the arms play the chief role and the disc appears as only 

 their united proximal ends (fig. 285). On the other hand, the disc may 

 increase at the expense of the arms, so that they form merely the angles of 

 a pentagonal disc (fig. 286). In both arms and disc two surfaces are 

 recognized, oral and aboral, which pass into each other, usually without 

 a sharp margin. In the normal position the oral side is downwards and 

 has the mouth in the centre and radiating from it to the tips of the arms the 

 five ambulacral grooves. Near the centre of the aboral surface is the 

 anus (when not degenerate) and excentric from it in an interradius is the 

 madreporite (in many-armed species two to sixteen interradii may have 

 madreporites). 



