672 MOLLUSCA [CEPHALOPODA. of the folds of the mantle to form a definitely -closed shell-sac ; (d) the secretion by these mantle-folds or walls of the shell- sac of additional laminae of calcareous shell- substance, which invest the original shell and completely alter its appearance (Spirulirostra, fig. 100, C; Belemnites); (e) the gradual dwindling and total disappearance of the original chambered shell, and survival alone of the calcare ous laminee deposited by the inner walls of the sac (Sepia, fig. 100, B) ; (/) the disappearance of all calcareous sub stance from the pen or plate which now represents the contents of the shell-sac, and its persistence as a horny body simply (Loligo, fig. 99); (g} the total disappearance of the shell-sac itself, and consequently of its pen or plate, nevertheless the rudiments of the shell-sac appearing in the embryo and then evanescing (Octopus). The early appearance of the sac of the mantle in which the shell is enclosed, in Dibranchiata, has led to an erroneous identifi cation of this sac with the primitive shell-sac of the archi- Mollusc (fig. 1), of Chiton (fig. 10, A), of Arion (fig. 69, D, a), and of the normally-developing Molluscan embryo (figs. 68 and 72***, sh). The first appearance of the shell- sac of Dibranchiata is seen in figs. 121 and 122, its forma tion as an open upgrowth of the centro-dorsal area of the embryo having been demonstrated by Lankester (34) in 1873, who subsequently showed (35) that the same shell-sac appears and disappears without closing up in Argonauta and Octopus, and pointed out the distinctness of this sac and the primitive shell-gland. The shell of the female Argonauta is not formed by the visceral hump, but by the enlarged arms of the foot, which are in life always closely applied to it. The shell of such Pteropoda as have shells (the Thecoso- mata) is excessively light, and fits close to the animal, no air-chambers being formed. It is important to note that in this division of the Cephalopoda there is the same tend ency, which is carried so far in the Dibranchiate Siphono- pods, for the mantle-skirt to be reflected over and closely applied to the shell (e.g., Cavolinia, figs. 79 and 80). But in Pteropoda there is no complete formation of a closed sac by the reflected mantle, no thickening of the enclosed shell, no dwindling of the original shell and substitution for it of a laminated plate. The variety of form of the glass-like shells of Pteropoda is a peculiarity of that Head, Foot, Mantle-skirt, and Sub-pallial Chamber. In the Pearly Nautilus the ovoid visceral hump is completely encircled by the free flap of integument known as mantle- skirt (fig. 91, d, e). In the antero-dorsal region this flap is enlarged so as to be reflected a little over the coil of the shell which rests on it. In the postero-ventral region the nap is deepest, forming an extensive sub-pallia! chamber, at the entrance of which e is placed in fig. 91. A view of the interior of the sub-pallial chamber, as seen when the mantle-skirt is retroverted and the observer faces in the direction indicated by the reference line passing from e in fig. 91, is given in fig. 101. With this should be com pared the similar view of the sub-pallial chamber of the Dibranchiate Sepia (fig. 103). It should be noted as a difference between Nautilus and the Dibranchiates that in the former the nidamental gland (in the female) lies on that surface of the pallial chamber formed by the dependent mantle-flap (figs. 101, g.n. ; 89, F), whilst in the latter it lies on the surface formed by the body-wall ; in fact in the former the base of the fold forming the mantle-skirt com prises in its area a part of what is unreflected visceral hump in the latter. The apertures of the two pairs of nephridia, of the vis- cero-pericardial sac, of the genital ducts, and of the anus are shown in position on the body-wall of the pallia! cham ber of Nautilus in figs. 101, 102. There are nine apertures in all, one median (the anus), and four paired. Besides these apertures we notice two pairs of gill-plumes which are undoubtedly typical ctenidia, and a short papilla (the nepli.p Fio. 101. View of the postero-ventral surface of a female Pearly Nautilus, the mantle-skirt (c) being completely reflected so as to show the inner wall of the sub-pallial chamber (drawn from nature by A. G. Bourne), a, muscu lar band passing from the mid-foot to the integument ; 6, the valve on the surface of the funnel-like mid-foot, partially concealed by the inrolled lateral margin of the latter ; c, the mantle-skirt retroverted ; an., the median anus ; x, post-anal papilla of unknown significance ; g.n., nidamental gland ; r.or. , aperture of the right oviduct ; l.ov., aperture of the rudimentary left oviduct (pyrifonn sac of Owen); neph.a., aperture of the left anterior nephridium; nepli.p, aperture of the left posterior nephridium ; viscper., left aperture of the viscero-pericardial sac ; olf, the left osphradium placed near the base of the anterior gill-plume. The four gill-plumes (ctenidia) are not lettered. osphradium) between each anterior and posterior gill-plume (see figs. 101, 102, and explanation). As compared with this in a Dibranchiate, we find (fig. 103) only four aper- visc.yt i: Fio. 102. View of the postero-ventral surface of a male Pearly Nautilus, the mantle-skirt (c) being completely reflected so as to show the inner wall of the sub-pallial chamber, and the four ctenidia and the foot cut short (drawn from nature by A. G. Bonnie), pe., penis, being the enlarged termination of the right spermatic duct ; l.sp., aperture of the rudimentary left spermatic duet (pyriform sac of Owen/. Other letters as in fig. 101. tures, viz., the median anus with adjacent orifice of the ink-sac, the single pair of nephridial apertures, and one asymmetrical genital aperture (on the left side), except in female Octopoda and a few others where the genital ducts and their apertures are paired. No viscero-peri cardial pores are present on the surface of the pallial
chamber, since in the Dibranchiata the viscero-pericardial