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. 3, is given in fig. 4. With this should be compared the similar view of the sub-pallial chamber of the Dibranchiate Sepia. 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 (fig. 4, g.n.; fig. 1, 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 comprises in its area a part of what is unreflected visceral hump in the latter.
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| Fig. 2.—Spirit specimen of female Pearly Nautilus, removed from its shell, and seen from the antero-dorsal aspect (drawn from nature by A. G. Bourne). | |
m, The dorsal “hood” formed by the enlargement of the outer or annular lobe of the fore-foot, and corresponding to the sheaths of two tentacles (g, g in fig. 6).
n, Tentacular sheaths of lateral portion of the annular lobe.
u, The left eye.
b, The nuchal plate, continuous at its right and left posterior angles with the root of the mid-foot, and corresponding to the nuchal cartilage of Sepia. |
c, Visceral hump.
d, The free margin of the mantle-skirt, the middle letter d points to that portion of the mantle-skirt which is reflected over a part of the shell as seen in fig. 1, b; the cup-like fossa to which b and d point in the present figure is occupied by the coil of the shell.
g.a. points to the lateral continuation of the nuchal plate b to join the root of the mid-foot or siphon. |
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| Fig. 3.—Lateral view of the same specimen as that drawn in fig. 2. Letters as in that figure with the following additions— | |
e, points to the concave margin of the mantle-skirt leading into the sub-pallial chamber.
g, The mid-foot or siphon.
k, The superficial origin of its retractor muscles closely applied to the shell and serving to hold the animal in its place. |
l, The siphuncular pedicle of the visceral hump broken off short.
v, v, The superior and inferior ophthalmic tentacles. |
The apertures of the two pairs of renal sacs, of the viscero-pericardial sac, of the genital ducts, and of the anus, are shown in position on the body-wall of the pallial chamber of Nautilus in figs. 4, 5. 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 osphradium) between each anterior and posterior gill-plume (see figs. 4, 5, and explanation). As compared with this in a Dibranchiate, we find (fig. 25) only four apertures, viz. the median anus with adjacent orifice of the ink-sac, the single pair of renal 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-pericardial pores are present on the surface of the pallial chamber, since in the Dibranchiata the viscero-pericardial sac opens by a pore into each nephridium instead of directly to the surface. A single pair of ctenidia (gill-plumes) is present instead of the two pairs in Nautilus. The existence of two pairs of ctenidia and of two pairs of renal sacs in Nautilus, placed one behind the other, is highly remarkable. The interest of this arrangement is in relation to the general morphology of the Mollusca, for it is impossible to view this repetition of organs in a linear series as anything else than an instance of metameric segmentation, comparable to the segmentation of the ringed worms and Arthropods. The only other example which we have of this metamerism in the Mollusca is presented by the Chitons. There we find not two pairs of ctenidia merely, but sixteen pairs (in some species more) accompanied by a similar metamerism of the dorsal integument, which carries eight shells. In Chiton the renal organs are not affected by the metamerism as they are in Nautilus. It is impossible on the present occasion to discuss in the way which their importance demands the significance of these two instances among Mollusca of incomplete or partial metamerism; but it would be wrong to pass them by without insisting upon the great importance which the occurrence of these isolated instances of metameric segmentation in a group of otherwise unsegmented organisms possesses, and the light which they may be made to throw upon the nature of metameric segmentation in general.
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| Fig. 4.—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, Muscular band passing from the mid-foot to the integument.
b, The valve on the surface of the funnel, 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.ov, Aperture of the right oviduct. |
l.ov, Aperture of the rudimentary left oviduct (pyriform sac of Owen).
neph.a, Aperture of the left anterior renal sac.
neph.p, Aperture of the left posterior renal sac.
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. | |
The foot and head of Nautilus are in the adult inextricably grown together, the eye being the only part belonging primarily to the head which projects from the all-embracing foot. The fore-foot or front portion of the foot has the form of a number of lobes carrying tentacles and completely surrounding the mouth (figs. 2, 3). The epipodia incline towards each other posteriorly so as to form an incomplete siphon (fig. 4), a condition which is completed and rendered permanent in the tubular funnel of Dibranchiata. The epipodial nature of the funnel is well seen in young embryos, in which this organ is situated laterally and posteriorly between the mantle and the foot.
The lobes of the fore-foot of Nautilus and of the other Cephalopoda require further description. It has been doubted whether these lobes were rightly referred (by T. H. Huxley) to the fore-foot, and it has been maintained by some zoologists (H. Grenadier, H. von