KONGL. SV. VET. AKADEMIENS IIANOL. BAND. 19. N:0 7. 77 



In like manner the conception of the costals as "genital plates» on a closer in- 

 spection ceases to be tenable. Subsequently to the appearance externally, in tlie young 

 Kchinid, of tin; niadreporic filter, when the reproductive glands approach maturity, 

 their efferent ducts pencti-ate from within the costals, and, in the adult state of the 

 great majority of species, a sexual pore is found in each of these. To this there are 

 but rare exceptions, such as Goniopygus, but in the Clypeastridaj not a few forms have 

 their sexual apertures outside the costals, in the respective interradia ^). In the En- 

 docyclic Gnathostotnes all the five costals are perforated, but four only in the earliest 

 among the Exocyclic, in Pygaster, the fifth having disappeared along with the costal 

 5, at the outbreak of the periproct. And, though in Oolitic species of Holectypus 

 the costal is restored, it is not until in the Cretaceous era that its pore reappears "). 

 just as in Discoidea, of early Cretaceous origin, it reverts ordy in the Turonian time, — 

 and this is the last instance known of its reinstatement, for among the Echinoneidas 

 and Cassidulidaj it never reappears. In these last-named the four sexual pores are far 

 asunder, owing to the central expansion of the madreporite, they are drawn near to- 

 gether latei-ally in the narrowed and lengthened calycinal system of the Collyrites and 

 in the Meridosternous Spatangi, such as Anancites, Hemipneustes and others, and they 

 are squared again in the shortened system af the Amphisternous Ethmophracti. In the 

 Spatangida; Ethmolysii, also, four is their normal number. In some cases, for instance 

 in Spatangus purpureus, they seem to make their appearance very irregularly, all four 

 being present in a specimen of the stage ram. 15 : 14, PL XVIIl, pg. 213, but wanting 

 in another of mm. 16:15, jig. 214, almost so even at mm. 19:18,/^. 216, highly 

 developed at mm. 23 : 22, jig. 218, while the two anterior are only indicated in a spe- 

 cimen of mm. 24:21. fig. 217, — anomalies that possibly arc apparent only, and may 

 depend upon difference of sex. During the growth, also, one or even two of the pores are 

 liable to give way in the struggle against the madreporite. In the costal 2, the old 

 site of the filter, this not seldom prevails and causes the sexual pore to abort, some- 

 times accidentally, as in Brissopsis lyrifera, PL XIX, fig. 229, but regularly in certain 

 species of Schizaster, as Sch. fragilis D. & K. '), Sch. Moselejd Al. Ag., in Tripylus, 

 and in Abatus cavernosus Phil., PL XVIlh fig. 220. In Abatus Philippii n. 'j, in 

 Moira atropos Lamk., and in some other species of Schizaster, as Sch. canaliferus Lamk., 

 Sch. japonicus ,\l. Ag., it has disappeared in the costal 3 also. But among the Spa- 

 tangida} the sexual organs are never deprived of their outlets in the costals 1 or 4 



Thus the madreporite, with its internal apparatus of cavities, passages and ca- 

 nals combined into a solid calcified structure, by its expansion and regressive move- 

 ment determines, in the Spatangidje, the closing up of one or more of the sexual 

 openings, and constantly that of the costal 5. In earlier types, when this same costal, 

 uppressed for a tim^ in consequence of the retrograde passage of the excretory ori- 



') Etudes, p. 80, I'l. XVI, tig. 13G. lb. p. 69. Cotteau Pekon et G.vutiuer, Ecliinides de I'Algerie, V, 



p. 223, PI. XVr, fig. 1—12. 

 ^) See above, woodcut p. 68. 



*) Etudes, pi. XII, fi;,'. 102. In Sell, gibbevuhis Ac. all four sexual pores are present. 

 «) lb. pi. XI, fig. 99. 



