MOLLUSCA 



135 



of the folds of the mantle to form a definitely-closed 

 shell-sac ; (il) the secretion by these mantle-folds or walls 

 of the shell-sac of additional laminre of calcareous shell- 

 substiince, which invest the original shell and completely 

 alter its appearance (Spirulirostra, fig. 100, C; Belemnites) ; 

 (c) the gradual dwindling and total disappearance of the 

 original chambered shell, and survival alone of the calcare- 

 ous laminre 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); (</) 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 Dibrauchiata, 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***, sfi). 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 

 group. 



Head, Foot, Mantle-skirt; and Sub-pallial Chrimlier. 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 

 flap is deepest, forming an extensive sub-pallial 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, V), 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 pallial 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 tico pairs of gill-plumes which 

 are undoubtedly typical ctcnidia, and a short papilla (the 



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). , muscu- 

 lar band passing from the mid-foot to the integument ; b, 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 ; a., the median anus ; 

 x, post-anal papilla of unknown significance ; 17.71., nidamental gland ; r.ov., 

 aperture of the right oviduct ; l.oi'., aperture of the rudimentary left oviduct 

 (pyriform sac of Owen); neph.a., aperture of the left anterior nephridium; 

 neph.p, aperture of the left posterior nephridium ; riscper., left aperture of 

 the viscero-pericardial sac ; olf, the left osphradium placed near the base of 

 the anterior gill-pluine. The four gill-plumes (cteuidia) 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- 



me suo-pamai cnamiier, ana Tin- lour cieniuia ;iliu uie loot cut snort tuiaun 

 from nature by A. G. Bourne). j>c., penis, being the enlarged termination 

 of the right spermatic duct ; l.sp., aperture of the rudimentary left spermatic 

 duct (pyriforni sac of Owen). Other letters as ill fig. 101. 



tures, viz., the median anus with adjacent orifice of the 

 ink-sac, the single pair of uephridial apertures, and one 

 asymmetrical genital aperture (on the left side), except in 

 female Octopoda and a few others where the genital 

 duets 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 



