234 PHYLUM ECHINODERMATA. 



The axial organ is well developed and lies close to the stone- 

 canal in the axial sinus. The dorsal end of it projects into a 

 sinus just below the madreporic plate. This is called the 

 madreporic vesicle, and is the right hydrocoel of the larva. The 

 axial organ consists largely of connective tissue, and, its wall 

 being folded, it appears to be penetrated by epithelial diverticula 

 of the body-cavity on the one hand and the axial sinus on the 

 other. It contains a number of cells which in the larva were 

 derived from a downgrowth into it of the genital rudiment.* 

 Its relation to the axial sinus is described on p. 231. 



They are dioecious. The gonads are typically five in number ; 

 but in many Spatangoids the number is reduced to 4, 3, or even 

 2, and in some Clypeastroids to 4. They are branched racemose 

 glands and are interradial in position. With a few exceptions 

 in Clypeastroids, their ducts open through the basal plates. 



The genital organs arise as they do in Asteroids as an out- 

 growth of the genital rudiment (p. 146), which, becoming enclosed 

 by a fold of the wall of the left coelomic sac, encircles the apical 

 pole of the animal and constitutes the genital rachis. The 

 genital organs themselves are developed as outgrowths of the 

 rachis. The part of the left coelom enclosed by the fold above 

 referred to becomes cut off from the rest and persists as the 

 aboral sinus (p. 146). 



The eggs are small and very numerous ; they are generally 

 discharged into the sea, where fertilization occurs. In a few 

 forms (species of Cidaris) the eggs become attached to the apical 

 part of the test amongst the spines and undergo their develop- 

 ment there. In some Spatangoids, some or all of the dorsal 

 parts of the ambulacra are sunk in and serve as brood pouches 

 (marsupia). 



The free larva has the pluteus form ; for a description of it 

 and of the main features of the development the reader is re- 

 ferred to pp. 140, 150. 



Echinoids have a considerable power of repairing injuries, 

 but not of forming new individuals from broken-off pieces. 

 Asexual reproduction is unknown in the group. 



The Endocydica for the most part creep upon a rocky bottom, 



* The genital rudiment is developed as a solid outgrowth of the epithelium 

 of the left posterior coelomic sac close to the septum separating it from 

 the left anterior coelom. 



