588 



LECTURE XXIII. 



each side, which lead to, and may admit sea-water into, the compart- 

 ments containing the posterior clusters of the venous follicles. 



The venous branches, from the labial and digital tentacles, and 

 adjacent parts of the head and mouth, terminate, with those from the 

 funnel, in the sinus, partly excavated in the body of the cartilaginous 

 skeleton, and in part continued round the oesophagus. From this 

 sinus, the great vena cava {Jig. 217, «) is continued, running in the 

 interspace of the shell-muscles on the ventral aspect of the abdominal 

 217 



Nautilus Pompilius. 



cavity, and terminating within the pericardium by a slight dilatation 

 (c), which receives by two veins {d) the blood from the different 

 viscera. The vena cava is separated by a layer of decussating mus- 

 cular fibres from the abdominal cavity, which closely adheres to the 

 parietes of the vein. There are several small intervals left between 

 the muscular fibres and corresponding round apertures {h) in the 

 membrane of the vein and peritoneum. This communication with 

 the diffused sinuses occupying the general abdominal cavity, is similar 

 to that already noticed in the Aplysia * : M. Valenciennes detected 

 the same structure in the specimen of the Nautilus dissected by him.j 

 The branchial circulation may be considered to commence, when 

 the blood again begins to move from trunks to branches, four of 

 which trunks are continued from the terminal venous sinus to con- 

 vey the carbonised blood to the four gills, of which there is a larger 

 and a smaller one on each side. Each pair of gills is connected by 

 a common peduncle to the inner surface of the mantle ; the larger 



r. 559. 



t Luc, cit. p. 287. 



