KONGL. SV. VET. AKADEMIEXS HAXDIJNGAR. BAND 19. N:0 7. 75 



trace of an intact margin, and even to detach one or other of its pores to the fiir- 

 tlier side, into the interradio,! area 5, /?</. 219. 



Tlie presence, outside tlie costal o and witliin the corresponding interradiura, of 

 a single pore of the niadreporic filter, at first seems nothing more than an accidental 

 anomaly. On a closer inspection, however, it looks otherwise. In a specimen of Bris- 

 sus canariensis H^eckel mf^. I observe a dense group of six pores Avithin the inter- 

 radiura 5, on the left from the costal, and a set of specimens just at hand of Briss- 

 opsis lyrifera presents not a few cases of the same disposition, PL. XIX, fig. 223 — 231. 

 In the adults of this well-known species the sexual apertures are normally four, 

 and one of them is in the costal "2. In a specimen of mm. 9 : 7, fiij. 223., the system 

 is made up of the five radials, the costals 1, 3, 4 distinguished by definite sutures, 

 and the costals 2 and 5 together M'ith the central ossicle united into one piece, the 

 madreporic filter here as elsewhere effacing the sutures. Only one minute pore is to 

 be seen, in the direction of costal 2. Next, in a specimen of mm. 11 : ^-fig- 224, one 

 more lias been added, in the central part; in specimens of mm. 15:12, 15:13, 16:13, 

 ////. 223 — 227, their number increases, occupying the centre and the costal o. Then, 

 while the central part contracts, the filter, always increasing the number of its pores, 

 expands, and forces the costal 5 backward, beyond the limits of the system ^). And while it 

 thus takes more room, as the animal grows, it is sometimes seen to prevent the sexual 

 organs from opening in the costal 2, fig. 229, sometimes to expand in 1 outside its 

 sexual pore, fig. 230, or sometimes to migrate, in no small number, across the limits of 

 the system and into the interradium 5, fig. 228, partly even into the ambulacrum I 

 /?'/. 231. In a hundred specimens of Brissopsis lyrifera taken at random, all from one 

 locality, I find ten presenting this anomalous disposition, the expelled pores being in 

 most cases on the right side, in the b series of the interradium. 



Thus the movement of the madreporic filter, the starting point and the terminus 

 of which have marked different geological epochs, is seen to take place in the living 

 animal within the brief space of a transitory stage of its development. In the adult 

 of Hemiaster and of all the other Spatangida? of early Mesozoic origin, the calyx 

 presents a structure essentially different from that of the calyx in the adult of a Spa- 

 tangus, or of any other form, of later or recent appearance, and very rarely a link of 

 connection is found between the two. But in the last-named of these groups an evo- 

 lutional process actually takes place, showing us how, in the individual, the calyx passes 

 from the one state into the other, and permitting us, however vaguely, to surmise 

 tlie nature of the modifications by means of which the same change may have been 

 brought about in the species, and the Ethmolysii made to take the place of the 

 Ethmophracti. The contending activities of the internal organs, of sensation, genera- 

 tion and circulation, which, after the removal of the excretory opening, in the ear- 

 liest Spatangida) still for a time combined to render permanent the site ofthemadre- 

 porite in the costal 2, and to hold back the restoration of the central disk and the 

 costal 5, entei'ed into a period of alterations that bj- degrees induced the setting free 



') Etudes, p. 1-2, pi. XII, Kg. 100, lol. 



