418 



COMPARATIVE ANATOMY 



CHAP. 



The madreporite of the primary (mediodorsal) stone canal. The simplest, and 

 no doubt also the most primitive condition is found in Pelagothuria and in certain 

 Elcisipoda, e.g. species of the genera Scotoplanes, Kolga, Parelpidia, Elpidia, Penia- 

 ijonc, and Benthodytes. In these the stone canal opens simply through a single 

 mediodorsal pore, which lies in front of the genital aperture (Fig. 357 A). In other 

 species of these genera and in species of Psychropotes, Lcctmogone, llyodccmon, more 

 than one madreporite pore is found, their number varying, according to. the species, 

 from two or three to fifty or more (Fig. 357, B). In other cases (species of the Elasipod 

 genera Irpu, Elpddia, Oneiroplmnta, Orphnurgus, Benthodytes, and the Molpadiidan 

 genera Trochostoma and Ankvroderina) the distal end of the stone canal still remains 



Fig. 357.— Diagrams illustrating tlie various relations existing between the stone canal 

 ana the madreporites in the Holotliurioiaea. 1, Body wall ; 2, cominencement of the radial 

 canal ; 3, ojsophagus ; 4, dorsal mesentery ; 5, stone canal ; 6, outer madreporite ; 6i, inner madre- 

 porite ; 7, genital aperture ; 8, genital duct ; 9, water vascular ring ; 10, Polian vesicle. 



embedded in the body wall, but it has lost the pore or pores which formed the com- 

 munication between it and the exterior. New pores therefore arise laterally at the 

 distal portion, which still lies in the body wall, and these now open communication 

 between the lumen of the stone canal and the body cavity, and make this widened 

 part of the stone canal into an inner madreporite (Fig. 357, C). Other Molpadiidcc 

 and the Synaptidcr and Dendrodiiruta differ from these last only in the fact that in 

 them the stone canal has become entirely detached from the body wall (Fig. 357, D). 

 In the Aspidoclurota, which also possess an inner madreporite, the latter appears 

 complicated, in that its pore canals do not open direct into the lumen of the stone 

 canal, but first into a collecting cavity, which in its turn communicates by means of 

 an aperture (occasionally through several) with the lumen of the stone canal. 



2. Eehinoidea (Fig. 358, 33).— In the Echinoidea, so far as is 



