xii PHYLUM MOLLUSCA 797 



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 coelome in the Dibranchiata has the the extent already 

 indicated (p. 766) 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 nephridia ; in Nautilus this com- 

 munication is absent, but the coelome opens on the exterior by 

 two symmetrical viscero-pericardial 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 mass, 

 oesophagus, stomach, intestine, salivary glands, and digestive gland 

 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 

 csecal appendage 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 

 Tetrabranchiata there are, as already stated, in accordance with 

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

 branchial hearts are absent. 



Nervous system and sense-organs. The ganglia of the 

 central nervous system are in all closely aggregated together round 

 the oesophagus, as already stated to be the case in Sepia ; and the 

 general disposition is the same as that described. In Octopus the 

 ganglia are much less sharply marked off. In Nautilus, as already 

 mentioned, there is less concentration, and distinct ganglia are not 

 recognisable. All the Dibranchiata possess highly developed eyes 

 similar to those of Sepia ; but in Nautilus the eyes are of a much 

 simpler character, each consisting of a sac opening on the exterior 

 by a small rounded aperture, lined internally by a two-layered 

 retina similar to that of Sepia, but without lens, vitreous humour, 

 or cornea. In the embryo of the Dibranchiata, the eye passes 

 through a stage in which it is in the condition of an open cup 

 similar to the adult eye of Nautilus. Osphradia are present, as 



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