. Echinoidea of the Swedish South Polar Expedition. Sea urchins. TH. MORTENSEN, (Schwed. Siidpolar-Exp. and younger. The largest specimens from the marsupium correspond very nearly to the stage of Abatus caveruosus described above (p. 76 PI. XVII. Fig. 9); the apical system is not quite so far on in its development, the anal plates have only just begun to appear; the genital plate 5 is distinct. Upon the whole it corresponds closely to that of A. cavernosus. Both mouth and anus are closed as yet. The terminal suckers are rather conspicuous. There is no trace of a peripetalous fasciole, and, si
. Echinoidea of the Swedish South Polar Expedition. Sea urchins. TH. MORTENSEN, (Schwed. Siidpolar-Exp. and younger. The largest specimens from the marsupium correspond very nearly to the stage of Abatus caveruosus described above (p. 76 PI. XVII. Fig. 9); the apical system is not quite so far on in its development, the anal plates have only just begun to appear; the genital plate 5 is distinct. Upon the whole it corresponds closely to that of A. cavernosus. Both mouth and anus are closed as yet. The terminal suckers are rather conspicuous. There is no trace of a peripetalous fasciole, and, since in Abatus cavernosus the fasciole is already distinct in specimens of a corresponding stage, it seems evident that a fasciole is never developed in this form. — It is, however, desirable that somewhat larger young ones should be examined for settling this important question definitely. It can scarcely be doubted that the further development is quite in accordance with that of A. cavernosus, with the ex- ception alone of the absence of a fasciole. — Pedicellariae have not appeared as yet in these young specimens. The specimen next in size is mm. long, 7 mm. broad and mm. high (PI. XI. Figs. 2, 4), its shape being thus, upon the whole, already like that of the grown specimens. The periproct has almost its definite place, only slightly more to the abactinal side. The ambulacra are as yet simple, without any indication of the later peta- loid condition; the pores are single. The apical system (Fig. 28) is interesting in showing the same feature as the type specimen of A. Lorioli, viz. the madreporite being separated from the right anterior genital plate by a trans- verse line, resembling thus a real central plate — though I do not mean to maintain its homology with the central (suranal) plate of Sa/enia, Ecliinus etc.; it reaches some- what beyond the posterior ocular plates, even a little more so than in the grown specimens (cf. Figs. 26—27). Also on the left s
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