Ce manuel traite des céphalopodes, une classe de mollusques qui comprend des espèces comme les calmars, les pieuvres et les seiches. La section met en évidence leur anatomie, y compris les coquilles chambrées de certaines espèces, et comment ces espèces varient en termes de développement des coquilles, des coquilles entièrement développées aux structures rudimentaires ou internes.
2175 x 1149 px | 36,8 x 19,5 cm | 14,5 x 7,7 inches | 150dpi
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. A manual of zoology. Zoology. V. CEPHALOPODA 341 chambered shells; but in other recent forms and in man)'extinct species the shell is more or less rudimentary. In Spirilla (the animals of which are extremely rare, the dead shells common) there is a similar chambered shell, buried in the mantle (fig. 363). Its position {entogastric) is the reverse of that of Nautilus. In the Dccapoda the equivalent of the shell is completely concealed in the back of the am'mal. In the Sepias it is a lamellar calcareous structure, the â vell-known cuttle bone; in the Loliginid;e it forms a 'pen' of purely organic nature (fig. 311, A). Like true shell these dorsal structures are products of the external epithelium, but the epithelium, the shell gland which forms them, has Ijecome folded in and the walls have united over it. The shell of Argonauta (fig. 363) is different. It occurs only in the female, is thin as paper, spirally coiled at the tip, and is only in part a secretion of the body, for a part of it is formed by two tentacles which are expanded for this purpose. Internal partitions are lacking, and this shell serves as a nest for the eggs. Most Octopoda also lack a shell. A word or two may be added to correlate the recent and fossil shells of the Dibranchiata, which are always internal and more or less rudimentary. The fossil Belemnites (fig. 354, C) had a chambered shell (phragmocone) perforated for the siphuncle. In front this is prolonged ventrally into a thin broad plate, the proosiracum, while behind it is inserted in a calcareous sheath, the guard or rostrum. From this, by compari- son with the fossil Bdosepia [B), it is seen that the cuttle bone of commerce (.4) is the anterior part of the chambered shell, its laminae being the partitions, while in the animal the rostrum and siphuncle are in part retained. On the other hand, comparison with the fossil Ostracoteuihis (D) shows that in Ommastrephcs (E) we have but a remnant of the phragmocone, while the bulk of the pen
