xii PHYLUM MOLLUSCA 743 



in the male, but the female has an external shell (Fig. 655) of a re- 

 markable character. This is a delicate spiral structure, the internal 

 cavity of which is not divided into chambers. It is not secreted 

 l>y the mantle like the shells of other Mollusca, but by the surfaces 

 of a pair of the arms ending in expanded disc-like extremities, 

 which become applied to its outer surface ; its chief function is to 

 carry the eggs. 



An internal cartilaginous skeleton is present not only in 

 Sepia and Nautilus, as already described, but in all the Cephalo- 

 poda. Such an internal skeleton occurs in other groups some 

 'Chsetopoda (p. 433) and Arachnida (p. 618), but attains a much 

 more elaborate character in the present group than in any other 

 Invertebrates. 



The plume-shaped gills, lodged in the mantle-cavity, are two 

 in all the Dibranchiata, as in Sepia. In the Tetrabranchiata there 

 are four gills, similar in general character to those of the Di- 

 branchiata. Osphradia are present at the bases of the gills only 

 in the former sub-class. 



The coelome in the Dibranchiata has the extent already 

 indicated (p. 715) in the case of Sepia, except that in the Octopoda 

 the oral part does not exist. In Nautilus it encloses, besides the 

 heart and gonad, the vena cava and a part of the glandular 

 appendages of the afferent branchial vessels. " In the Dibranchiata 

 the pericardial portion communicates with the nephridia ; in 

 Nautilus this communication is absent, but the coelome opens on 

 the exterior by two symmetrical orifices placed at the side of the 

 openings of the aboral nephridia. 



Alimentary Organs. Jaws similar to those of Sepia are 

 present in all the members of the class ; in Nautilus, instead of 

 being completely horny, they are partly calcified. Buccal cavity, 

 oesophagus, stomach, intestine, salivary glands, and liver are all 

 of the same general character throughout all the members of the 

 class. In some of the Dibranchiata, such as Octopus, there are 

 two pairs of salivary glands. In Nautilus the salivary glands are 

 absent, so far as known, the oesophagus is^ dilated to form a sort 

 of crop, and the stomach is gizzard-like. In that genus also the 

 ink-gland, general in the Dibranchiata, is absent, and there is a 

 a3cal appendage to the intestine ; the liver is four-lobed, each 

 lobe having its duct. The so-called pancreas, described in Sepia, 

 is similarly developed in all the Dibranchiata, and is present also, 

 though only feebly developed, in the Tetrabranchiata. 



Heart and vascular system are well developed in the 

 Cephalopoda, and their structure and arrangement closely corre- 

 spond with what has been described in Sepia, except that in 

 Tetrabranchiata there are, as already stated, in accordance with 

 the double number of gills, four auricles instead" of two, and 

 branchial hearts are absent. 



