. Outlines of zoology. r in which it hasbeen living. All the compart-ments are in communication bya median tube of skin—thesiphuncle—which is in part cal-careous. It has been suggested that each septum shutting off anair-containing chamber is formedduring a period of quiescence,probably after the reproductiveact, when the visceral mass ofthe Nautilus may be slightlyshrunk, and gas is secreted fromthe dorsal integument so as tofill up the space previouslyoccupied by the animal. There can be no confusion between the beautiful shell ofthe cuttlefish called the paper Nautilus (Argonauta argo)and t
. Outlines of zoology. r in which it hasbeen living. All the compart-ments are in communication bya median tube of skin—thesiphuncle—which is in part cal-careous. It has been suggested that each septum shutting off anair-containing chamber is formedduring a period of quiescence,probably after the reproductiveact, when the visceral mass ofthe Nautilus may be slightlyshrunk, and gas is secreted fromthe dorsal integument so as tofill up the space previouslyoccupied by the animal. There can be no confusion between the beautiful shell ofthe cuttlefish called the paper Nautilus (Argonauta argo)and that of our type. For it is only the female Argonautwhich bears a shell; it is not chambered, and is a shelterfor the eggs—a cradle, not a house. It seems to beformed by two of the arms. It is instructive to compare the Nautilus shell with thatof some Gasteropods, for there also chambers are occasion-ally formed. But these arise firom ofan originally continuous spiral. The Gasteropod shell is. Fig. 232.—Section of shell ofNautilus.—After Lendenfeld. 43° PHYLUM MOLLUSC A. usually unsymmetrical, and the foot {ventral) is turnedtowards the internal curve of the coil, while in Nautilusthe dorsal part of the animal is towards the internalsurface of the chamber.
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
