684 ANAL SYSTEM. 



species. The membrane is covered by large, more or less polygonal plates, 

 becoming smaller towards the anal opening. As is the case in the Diadema- 

 tidae, the anal opening is sometimes placed at the extremity of a tube 

 projecting beyond the level of the test (PI. XVII. f. 4, 6). In one of the 

 families of Spatangoids, the Leskiadae, we find a limited number of anal 

 plates; in Paleostoma the anal plates (PI. XXXII f. 15) form a conical 

 projection, and are not arranged, as in the other Petalosticha, in concentric 

 rows decreasing in size towards the anal opening. See Part III. p. 582. 



In the Salenidae the larger plates covering the anal system become sol- 

 dered to the plates of the genital ring, while the minute plates which imme- 

 diately surround the anal opening are movable and do not differ from the 

 anal plates of the other Desmosticha. See PL III/. //. ami p. 258 of Part 

 II., for the description and discussion of the relationship of the apparently 

 abnormal apical system of this family. 



Among Spatangoids proper we never find more than four genital openings. 

 The central plate, which carries the madreporic body, becomes completely 

 united with the fifth imperforate genital plate. We thus find the structure 

 of the apical system passing through its simplest form as typified among 

 the regular Echini, in which the anal system forms a component part of the 

 genital and ocular systems, to the anal system of the Clypeastroids and the 

 Spatangoids, which becomes isolated from the genital and ocular rings; in 

 the former we still have left what we may consider the analogue of the large 

 primary anal plate of young regular Echini, which becomes the madreporic 

 plate, and is isolated from the genital plate ; this may be perforated or not : 

 in Spatangoids this genital plate never has a genital opening, and always 

 carries the madreporic body. 



The position of the madreporic body cannot, I think, be taken to indicate, 

 either in the regular Echini or in starfishes and Ophiurans, the position of an 

 anterior extremity. If we examine the position which the madreporic body 

 holds with reference to the winding of the alimentary canal in Spatangoids 

 and Clypeastroids, we find that it may be either to the right or to the left of 

 the longitudinal axis, as defined by the position and direction of the ali- 

 mentary canal, leading from the mouth either towards the anterior or towards 

 the posterior extremity of Spatangoids and Clypeastroids, as fixed by the 

 odd anterior ambulacrum. See PI. XXXII. f. 5, 11, in which the oesophagus, 

 when leaving the actinostome, trends in one case in the direction of the odd 

 anterior ambulacrum, and in the other in the direction of the odd posterior 

 interambulacral space. 



