CIRCULATION IN MOLLUSCA. 



251 



takes place nearly on the same general plan as in Fishes ; the 

 heart having.two cavities, and the whole of the blood travers- 

 ing "both the respiratory and the systemic vessels, between 

 each time of its leaving the heart and returning to it again. 

 But this heart is systemic, and not pulmonary ; for it receives 

 the arterial blood from the gills, and transmits it to the great 

 systemic artery ; and after the blood has been rendered venous 

 by its passage through the tissues of the body, it enters the 

 channels which distribute it to the gills, before being again 

 subjected to the action of the heart. The accompanying figure 

 (fig. 139) of the circulation in the Doris (a kind of sea slug) 



Fig. 139. CIRCULATING APPARATUS OF DORIS. 



will serve to show the general distribution of the vessels in 

 this group. The heart consists of the ventricle a, whence 

 issues the main artery I ; and of a single or double auricle c, 

 in which terminate the veins, d, of the branchial apparatus e. 

 The aerated blood which these convey to the heart, is trans- 

 mitted by it, through the artery 6, to the system at large ; and 

 from this it is collected, in the* state of venous blood, by the 

 sinuses which terminate in the large trunk //. By this trunk it 



