. Outlines of zoology. Zoology. 382 MOLLUSCA. is called " pearly" on account of the appearance of the innermost layer of the shell. This is exposed after the soft organic stratum and the median layer which bears bands of colour have been worn away, or dissolved in a dolphin's stomach, or artificially treated with acid. The beautiful shell is a spiral in one plane, divided into a set of chambers, in the last of which the animal lives, while the others contain gas. The young creature inhabits a tiny shell curved like a horn; it grows too big for this, and proceeds to enlarge its dwelli
. Outlines of zoology. Zoology. 382 MOLLUSCA. is called " pearly" on account of the appearance of the innermost layer of the shell. This is exposed after the soft organic stratum and the median layer which bears bands of colour have been worn away, or dissolved in a dolphin's stomach, or artificially treated with acid. The beautiful shell is a spiral in one plane, divided into a set of chambers, in the last of which the animal lives, while the others contain gas. The young creature inhabits a tiny shell curved like a horn; it grows too big for this, and proceeds to enlarge its dwelling, meanwhile drawing itself forward from the older part, and forming a door of lime behind it. This process is repeated again and again; as an addition is made in front, the animal draws itself forward a little, and shuts ;^v • , off a part of the chamber in 11 Wp|K^ which it has been living. Thus §§j||PI;Jg5^| the compartments are not suc- cessive chambers, but fractions of successive chambers, aban- doned and partitioned off as more space was gained in front. All the compartments are in communication by a median tube of skin—the siphuncle—which is in part calcareous. It has been suggested that " each septum shutting off an air-containing chamber is formed during a period of quiescence, 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 at all like that of the Nautilus is Spirula. In it 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). Fig. 167.—Section of shell of nautilus. — After Lenden- Please note that these images are extracted from sca
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Keywords: ., bookauthorth, bookcentury1800, bookdecade1890, booksubjectzoology
