KONGL. SV. VET. AKADEMIENS HANDL. BAND 19. N:() 7. 67 



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

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

 minute ossicles, in the centre of which is the excretory opening. In this strictly ortho- 

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

 the vectilinear margins of the costals, while in most of the Echinidse the partial i'e- 

 sorption of these margins, corabined 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 Echinidaä, and most probably also in 

 that of the Cidai'ida^, during the transitory astomous and aproctic stage of the verj' 

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

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

 at least, upon which its place is iilled 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 the 

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

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

 portion of the respective costals, which necessarily has been remo- 

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



111 1 1 1 • 1 il • i -il il Salenia hastigera Al. Ag. 



ner replaced by an anal membrane, wnich tnus coexists with the Caiycina! svstem. 



nearly intact central disk. The difierence is, that, in the Cidaridae 



and the Echinidge, unlike what takes place in all the SaleniadaB, the excretory opening 

 breaks out, not wholiy 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 excretory opening continues endocyclic, a certain degree of sta- 

 bility maintains in the disposition of the calycinal system. Costals and radials are 

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

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

 the sexual organs open regularly 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 calycinal system, balance each other in an equal 

 manner. In the Cidarida3 and the whole numerous group of Echinidas it has continued 

 so to the present day. 



But this state of stability in the calycinal system is broken, as soon as one or 

 the other of the organs it covers begins to move^). The exci-etory opening is the 

 first to alter its position, it is followed by the madreporite and the sexual yjores, but 

 the eyes remain stationary. Already in the time of the Lias there existed, at the side 

 of Cidaridaä, Echinidie and Saleniadte, the group of the Echinoconidaj^), true Gnatho- 



') Études, p. 76. 

 2) Ib., p. 79. 



