. Outlines of zoology. Zoology. 384 MOLLUSC A. cence, probably after the reproductive act, when the visceral mass of the Nautilus may be slightly shrunk, and gas is secreted from the dorsal integument so as to fill up the space previously occupied by the ; The only other living Cephalopod which has a shell like that of the Nautilus is Spirilla. In this form the shell is again chambered and spirally coiled in one plane. But it is without a siphuncle, and lies enveloped by folds of the mantle. There can be no confusion between the beautiful shell of the cuttlefish called the paper Na
. Outlines of zoology. Zoology. 384 MOLLUSC A. cence, probably after the reproductive act, when the visceral mass of the Nautilus may be slightly shrunk, and gas is secreted from the dorsal integument so as to fill up the space previously occupied by the ; The only other living Cephalopod which has a shell like that of the Nautilus is Spirilla. In this form the shell is again chambered and spirally coiled in one plane. But it is without a siphuncle, and lies enveloped by folds of the mantle. There can be no confusion between the beautiful shell of the cuttlefish called the paper Nautilus {Argonauta argo) and that of our type. For it is only the female Argonaut which bears a shell, it is not chambered, and is a shelter for the eggsâa cradle, not a house. It is usually stated to be formed by two of the arms, but it seems doubtful whether it is not in reality due to the activity of the mantle. It is instructive also to com- pare the Nautilus shell with that of some Gasteropods, for there also chambers may be formed. But these arise from secondary alterations of an ori- ginally continuous spiral, and the resemblance is never very striking. The fresh water snail Planorbis has an unchambered shell spirally coiled in one plane, but in this and in similar Gasteropods, the foot is turned towards the internal curve of the coil, while that of Nautilus is directed externally. There are only about half a dozen living species o{Nautilus, but there are many hundred fossils of this and allied genera. This list is usually swelled by the addition of the extinct Ammonites, but there are some reasons for believing that these belong to the cuttlefish section of Cephalopods. The following table states the chief points of distinction between Nautilus and the other series of Fig. 123. of Nautilus. FELI).) â Section of shell (After Lenden-. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability -
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
