. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 518 CEPHALOPODA. In the first of these, which is most closely allied to the Gasteropodous Mol- lusks, the branchias are four in number, and the order is therefore termed Tetru- branchinta : in the higher division, which approaches nearest to the Vertebrate ani- mals, the branchiae are two in number, and the order is called DibrancAiata. Order I. TETRABRANCIIIATA. Syn. Polythalamaces, Blainville; Sipho- nifera, D'Orbigny; minus the Spindida: and BelemnitidtE. The Tetrabranchiate Cephalopods, of which the Pearly Nautilus (
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 518 CEPHALOPODA. In the first of these, which is most closely allied to the Gasteropodous Mol- lusks, the branchias are four in number, and the order is therefore termed Tetru- branchinta : in the higher division, which approaches nearest to the Vertebrate ani- mals, the branchiae are two in number, and the order is called DibrancAiata. Order I. TETRABRANCIIIATA. Syn. Polythalamaces, Blainville; Sipho- nifera, D'Orbigny; minus the Spindida: and BelemnitidtE. The Tetrabranchiate Cephalopods, of which the Pearly Nautilus (Jig. 205) may be regarded as the type, are provided with a large external uni- valve shell, sym- metrical in form like the body of the animal which it protects, straight, or con- * voluted on a ver- tical plane, and divided by a se- ries of partitions («, a) into nume- rous chambers (b, 6), of which the last-formed (/>') is The Pearly Nautilus, Nautilus Pompilius, Linn. the largest, and alone contains the body of the animal: a dilatable and contractile tube (c, c) is continued from the posterior part of the M. D'Orbigny believes to be constructed by mol- luscous animals of a grade of organization which entitles them to rank with the Cephalopodous class. For this group of animals M. De Haan has pro- posed the name of Asiphonoidea ; but M. D'Orbigny, observing that the chambers of their shells com- municate together by means of one or more fora- mina, has substituted the positive term Foraminifera, and they are placed by Cuvier at the end of the Cephalopodous class under that denomination in the last edition of the Regne Animal. Strong evidence has, however, been recently ad- duced to prove that these minute shells owe their existence to animals which have no pretensions to rank with the Cephalopods ; but before we give the account of M. Dujardin, who is the author of this view, we shall first quote M. D'Orbigny's own description of the animal of the shells, the strucr
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