212 INVERTEBRATE ANIMALS. 



the pvirpose of conveying the sperm-cells to the female. For the 

 above reason, the general name of " hectoootylus " is given to the 

 detached reproductive arm of any of the Cuttle-fishes (fig. 150, h). 



The shell of the Cepludopoda is sometimes external, sometimes 

 internal. The internal skeleton is seen in most of the Cuttle-fishes, 

 iu which it is known as the " cuttle-bone " or " pen." It may be 

 either horny or calcareous, and it is sometimes complicated by the 

 addition of a chambered portion. The only living Cephalopods 

 which are provided with an external shell are the Paper Nautilus 

 {Aiyonauta) and the Pearly Nautilus (Xnutilvs jjiimpUms); but not 

 only is the structure of the animal different in each of these, but 

 the nature of the shell itself is entirely different. The shell of the 

 Argonaut (fig. 151) is coiled into a spiral, but it is not divided into 

 chambers, and it is secreted by the webbed extremities of two of the 

 dorsal arms of the female. These arms are bent backwards, so as to 

 allow the animal to Ii\'e in the shell ; but there is no organic connec- 

 tion between the shell and the body of the animal. The shell of the 

 Pearly Nautilus, on the other hand, is secreted by the mantle, and 

 is organically connected to the animal. It is coiled into a spiral 

 (fig. 155), but it differs from the shell of the Argonaut in being 

 divided into a series of chambers by means of shelly partitions, 

 which are connected together by a tube or " siphuncle," the animal 

 itself living in the last and largest chamber only of the shell. 



The Cephalopoda are divided into two extremely distinct and 

 natural orders, termed respectively Dihraiichiata and Tetrabranchiata, 

 according as they have two or four gills or branchioe. 



The Dihrdiii-Jiirtta comprise the Cuttle-fishes, Squids, Calamaries, 

 and Paper Nautilus, and they are characterised by being almost 

 invariably destitute of any external shell ; by never having more 

 than eight or ten arms, which ai'e always furi^ished with suckers ; 

 by having only two gills, which are provided with "branchial 

 hearts"; by the possession of an "ink-ba;,'"; and by the fact that 

 the "funnel" forms a complete tube. They are divided into two 

 sections — Octopodti and Dernpnda — according as they have only 

 eight arms (fig. 150), or eight arms A^ith two additional longer jiro- 

 cesses or "tentacles" (fig. 15.3). Amongst the <hti>iioda are the 

 Pajjer Nautilus and the Poulpes (Octopus). The Paper Nautilus is 

 found in the warmer seas of various parts of the world, generally 

 floating at the surface. The two sexes differ, as already said, 

 greatly in external ajjpearance. The female (fig. 151) inhabits a 

 beautiful one-chambered cell, which is secreted by the webbed ex- 

 tremities of two of the dorsal arms. The shell is not in any way 

 attached to the body of the animal, but the webbed arms are turned 

 backwaids, and the animal sits in the shell with the funnel turned 



