440 Prof. S. Lov^n on the Structure of the Echinoidea. 



membrane. This change of position depends, no doubt, upon 

 the circumstance that the periproctium, by the resorption, espe- 

 cially, of the vertical plates 1 and 5 and by stretching, is 

 enlarged more rapidly and strongly than the breadth of the 

 vertical plates increases, and that the eye-plates, on which the 

 growth of the ambulacra reacts as a vis a tergo^ are driven into 

 its median space. Vertical plate 2, which contains the madre- 

 porite, is enlarged more than the others, and keeps back eye- 

 plates II. and III. 



The anal aperture is produced in the same way in the Spa- 

 tangid^e and Echinidge. Quite young individuals of the former 

 have the periproctium much more dorsal than older ones, as is 

 the case in a high degree in a Spatangus purpureus^ 2*025 

 millims. in length ; this species is destitute of the peripetalous 

 fasciola. So early as this the periproctium is nearly round, with 

 the anus about in the middle. The anal membrane is closely 

 covered with thin imbricated scales, which form circles : — 

 the innermost one of long, narrow, pointed lamella, connivent 

 around the anus ; and around this one or more, consisting of 

 larger triangular scales ; the outermost a circle of still larger 

 plates. Of these last, in many genera, the adoral grow pre- 

 ponderantly, so that the periproctium is gradually elongated, 

 and the anus comes to be situated more excentrically in an 

 aboral direction. It is otherwise in the Echinidag. 



In his memoir on the Echinoidea collected by Pourtales in 

 the great depths between Florida and Cuba, Alexander 

 Agassiz states that in a very early stage, when, however, the 

 mouth with its jaws is already developed, " the anal system of 

 the Echinidffi is limited to a single subanal plate, which makes 

 its appearance before the genital and ocellar plates, and long 

 remains more prominent than the other plates, which are 

 added to complete the enlarged anal system " *. The penta- 

 gonal or somewhat rounded space which is enclosed by the 

 five vertical plates is embraced at this stage by a single un- 

 paired disk. Soon afterwards, in a young Toxopneustes dro- 

 hachensisj place begins to be prepared for the anus ; but 

 this occurs not in the middle of the disk, but excentrically and 

 outside of it. The margins of vertical plates 1 and 5 are 

 absorbed, and between them and the central disk an interspace 

 is produced which is occupied by the soft general integument. 

 In this is formed a pair of free, rounded, oblong, calcareous 

 pieces, which do not coalesce with the disk. Whilst the peri- 

 proctium enlarges so that it becomes oval in an oblique direc- 

 tion, and the disk is raised a little at its free margin, although 



* « Contributions to the Fauna of the Gulf-stream ' &c. pp. 281, 284, 285. 

 From ' Memoirs of the American Academy/ ix. p. 12. 



