KONGL. SV. VET. AKADEMIENS HAXDL. HANI) 19. N:() 7. 67 



marked. One, however, of its three eonstituent elements is no longer persistent in 

 the adult; the central disk is substituted by a pliant membrane paved with numerous 

 minute ossicles, in the centre of which is the exci :tor\' (jpening. In this strictly ortho- 

 proctic group the outline of this membrane still remains pentagonal, as determined by 

 the rectilinear margins of the rostals, while in most of tiic Echinidte the partial re- 

 sorption of these margins, combined with the excentric position of the excretory open- 

 ing, tends to conceal, in no small degree, the original existence of a central disk. 

 Now we know, however, that in the calyx of the Echinidas, and most probably also in 

 tliat of the Cidarida', during the transitory astomnis and aproctic stage of the very 

 young animal, the central disk exists in its legitimate position relatively to the other 

 constituents of the system, but onl_y for a short time, and soon to be resorbed, in part 

 at least, upon which its place is tilled by the anal membrane. Its 

 .structural and morphological identity with the central disk of the 

 Saleniada3 cannot be doubted, and, like that, it is not in the remo- 

 test manner referable, morphologically or physiologically, under tlie 

 appellation of an anal or subanal supplementary plate, to the di- 

 gestive apparatus, indeed no more .so than, in the Saleniadaj, tiie 

 portion of the respective co.stals, which necessarily has been remo- 

 ved in behalf of the formation of the periproct, and in like man- 



, T , , , I'll • i -J.! J.1 Salenia hastigera At.. Ag. 



ner replaced by an anal membrane, which thus coexists with the Caiycinai system. 



nearly intact central disk. The difference is, that, in the Cidaridte 



and the Echinidae, unlike what takes place in all the Saleniada>, the excretory opening 

 breaks out, not wholly or partially outside the central disk, but from under it, and thus 

 induces its destruction as such. The anal membrane supplying its place may possibly 

 be a dependency of the perisome. 



As long as the excretorv opening continues endocyclic, a certain degree of sta- 

 bility maintains in the disposition of the caiycinai sj^stem. ("ostals and radials are 

 constantly present in the normal number of five, and distinct sutures mark their mu- 

 tual limits. The Avater-pores penetrate the costal 2 alone and very rarely exceed it, 

 the sexual organs open regularlv by a pore in each costal, and there is an orbit, sim- 

 ple or double, in every radial. It is evident that the internal organs, in sharing among 

 themselves the constituents of the caiycinai system, balance each other in an equal 

 manner. In the Cidarida' and the whole numerous group of Echinida? it has continued 

 so to the present day. 



But this state of stability in the caiycinai system is })roken, as soon as one or 

 the other of the organs it covei's begins to move'). Tlie excretory opening is the 

 first to alter its position, it is followed by the madre])orite and the sexual pores, but 

 the eyes remain stationarv. .\lready in the time of the Lias there existed, at the side 

 of Cidarida}, Echinid;e and .Saleniada-. tlic group of the Echinoconida^ '), true Gnatho- 



') Etudes, p. 76. 

 2) II)., p. 79. 



