SANGUIFEROUS SYSTEM. 465 



The venous blood collected from all parts of the system in 

 the aplysia is transmitted to the branchial arteries by two 

 wide vense cavse, with thin membranous parietes ; and on 

 opening these two venous trunks, superficial depressions are 

 observed on their interior thin coats, corresponding with the 

 interstices of the surrounding muscular bands of the abdomen. 

 These depressions perceived on the serous lining of the 

 great systemic veins of the aplysia have been supposed to be 

 remarkable perforations leading into the general cavity of the 

 peritoneum, and probably serving the function of emunctory or 

 absorbent orifices, and the same anomalous structure has 

 been ascribed to the great systemic veins of the nautilus. 

 The serous lining of these wide veins of the aplysia, however, 

 was shown by Meckel to be entire, as in other animals. In 

 the testaceous and naked pulmonated gasteropods, where 

 there is no canal nor syphon on the left side, and in which 

 the respiratory sac, though opening on the right side, ex- 

 tends across the median plain, the two cavities of the heart 

 are directed transversely to the left side, between the pulmo- 

 nary cavity and the large muciparous gland, and the aorta 

 immediately divides into two great trunks, which send 

 branches to the abdominal viscera and the muscular parts. 

 The auricle is perforated by the two trunks of the pulmo- 

 nary veins, and two semilunar valves are seen in the ventricle 

 at the auriculo -ventricular orifice. The inferior of the two 

 aortic trunks is distributed chiefly on the liver and on the 

 ovary; the ascending trunk gives a branch to the sto- 

 mach and intestine, the right tentaculum, the dart-gland, 

 and the genital ducts ; another to the stomach, the sali- 

 vary glands, and oesophagus, and the continuation of the 

 trunk, after furnishing a branch to the left tentaculum, turns 

 backwards to spread its terminal branches on the large mus- 

 cular foot. The venous blood collected from the system into 

 a single vena cava, is transmitted by the ramifications of two 

 pulmonary arteries over the interior surface of the respira- 

 tory sac, and the capillaries reunite to form the two pulmo- 

 nary veins which enter the auricle of the heart. 



The structure and position of the heart, and the general 

 distribution of the blood-vessels, appear to be the same in 

 the pteropods as in the higher gasteropodous mollusca ; the 

 venous blood of the system is propelled through the external 



PART V. H H 



