538 



CEPHALOPODA. 



served in the class. Here the biliary ducts, as 

 soon as they emerge from the liver, branch out 

 into an arborescent mass of larger and more 

 elongated follicles than those constituting the 

 hepatic parenchyma; these ramifications extend 

 full half an inch from the hepatic duct, and 

 conceal the upper halves of both the stomach 

 and pyloric appendage. 



Organs of Circulation. Prior to the dis- 

 section of the Nautilus Pompilius the Ce- 

 phalopods were regarded as having three dis- 

 tinct hearts, a peculiarity which is not found in 

 the circulating system of any other class of 

 animals. In the Nautilus, however, there is 

 but one ventricle, which is systemic, as in the 

 inferior Mollusks ; and the three hearts are, 

 therefore, characteristic only of the Dibran- 

 chiate or higher order of Cephalopods. 



These differences in the circulating system of 

 the two orders are accompanied with equally well 

 marked modifications of the respiratory organs; 

 and hence the primary divisions of the class 

 are each distinguished by characters of equal 

 value, and derived from modifications of those 

 organs which afford the most natural indica- 

 tions of the corresponding groups in the other 

 classes of the Molluscous division of Inverte- 

 brate animals. 



In the Nautilus the veins which return the 

 blood from the labial and digital tentacles and 

 adjacent parts of the head and mouth, termi- 

 nate in the sinus excavated in the substance of 



the cephalic cartilage. From this sinus the great 

 anterior vena cava (p, Jig. 224) is continued, 

 running in the interspace of the shell-muscles 

 on the ventral aspect of the abdominal cavity, 

 and terminating in a sinus (6) just within the 

 pericardium, where it receives the venous 

 trunks of the viscera. (These are indicated by 

 bristles in the figure.) 



The structure of the vena cava is very remark- 

 able ; itisof aflattened form, being included be- 

 tween a strong membrane on the lower or ventral 

 aspect, and a layer of transverse muscular fibres, 

 which decussate each other on theupperor dorsal 

 aspect; both the membrane and the muscle 

 pass across from the inferior margin of one 

 shell-muscle to the other; they consequently 

 increase in breadth as those muscles diverge, 

 and complete the parietes of the abdomen on 

 the ventral aspect. The vein, however, main- 

 tains a more uniform calibre by its proper 

 internal coat, leaving a space on either side 

 between the membrane and muscle. The ad- 

 hesion of the proper membrane to the muscular 

 fibres is very strong, and these, though ex- 

 trinsic to the vessel, form part of its parietes 

 on the dorsal aspect. There are several small 

 intervals left between the muscular fibres and 

 corresponding round apertures (a') in the mem- 

 brane of the vein and contiguous peritoneum, by 

 which the latter membrane becomes continuous 

 with the lining membrane of the vein : from 

 this structure it would seem that the blood 



Fig. 224. 



Circtdating and Respiratory Organs, Nautilus Pompilius. 



