xn 



PHYLUM MOLLUSCA 



773 



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

 poda. Such an internal skeleton occurs in other groups some 

 Chsetopoda (p. 462), Crustacea, and Arachnida (p. 651) 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 number in all the Dibranchiata, as in Sepia. In the Tetra- 

 branchiata there are four gills, similar in general character to 

 those of the Dibranchiata. 



The ccelome in the Dibranchiata has the extent already 

 indicated 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, a part of the glandular appendages of the afferent 

 branchial vessels. In the Dibranchiata the pericardial portion 

 communicates with the renal sacs ; in Nautilus this communication 

 is absent, but the coelome opens on the exterior by two symmetrical 

 viscer o-peri- 

 cardial orifices 

 placed at the side 

 of the openings 

 of the aboral 

 kidneys. 



Alimentary 

 Organs. Jaws 

 similar to those 

 of Sepia are pre- 

 sent in all the 

 members of the 

 class ; in Nauti- 

 lus, instead of 

 being completely 

 horny, they are 

 partly calcified, 

 salivary glands, 

 general charm-in 



FIG. 705. Shell' of Argonauta argo. 



Buccal mass, oesophagus, stomach, intestine, 

 and digestive gland are all of the same 

 throughout all the members of the class. 

 In some of tin*. Dihni.nrhia.ta, such as Octopus, tlio.ro arc two 

 pairs of salivary glands. In Nautilus the salivary glands are 

 absent, so far as known, the (esophagus 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 

 caeca] append age to the intestine ; the digestive gland 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 



