MADREPORIC BODY. 



The function of the madreporic body is to introduce water into the water 

 system through the stone canal. The madreporic body occupies, however, a 

 larger space than the termination of the stone canal, so that a large portion 

 of the pmcs lead directly into the cavity of the body, and through its open- 

 ings currents can flow in or out. The water is either forced in or out accord- 

 ing to the greater or smaller space occupied by the ovaries at different times, 

 by the Idling or emptying of the alimentary canal, by the expansion or con- 

 traction of the ambulacral vesicles, all these conditions change the state of 

 equilibrium and produce currents of water either inward or outward through 

 the madreporic body. In the Desmosticha and Clypeastroids, where we have 

 an actinal membrane of considerable size, the power of increasing the capacity 

 of the interior of the test is very marked. We frequently find our common 

 sea-urchins | Toxopneustes, Strongylocentrotus. Diadema. Hipponoe), when 

 alive, with actinal membranes distended to an extraordinary degree, project- 

 ing far beyond the general level of the aetinostome. It is probable that 

 there also exists a connection between the water system, the circulatory sys- 

 tem, and the perivisceral cavity of the test ; but. however probable this con- 

 nection may be from some of the phenomena observed (the presence of pig- 

 ment cells in the water of the interior), the actual connection is not yet 

 proved. Hoffmann has made similar observations, but has not been able to 

 trace the direct communication. I have seen spermatozoa escape through 

 the madreporic body of our common sea-egg (S. Drobachiensis), and this 

 appears the only way they can escape when, through accident or otherwise, 

 they fall into the interior cavity by the breaking of the walls of the sper- 

 matic sacs. 



The madreporic body, as has been stated before, occupies a part of one of 

 the genital plates ; it is entirely within the genital ring in the Desmosticha. 

 It is disconnected from the genital plates in many of the Clypeastroids, occu- 

 pying the central part of the apical system, and it is connected with the right 

 anterior genital plates in the Petalosticha, while at the same time it fills a 

 greater or less space of the intergenital apical system. 



