MECHANISM OF THE CIECFLATION. 



63 



times veins exist in some parts of the body, whilst elsewhere 

 in the same animal their place is supplied by venous canals, 

 having no proper tunic, but consisting merely of the inter- 

 organic lacunae, or the large cavities of the body, as the 

 abdominal cavity (m Fig. 44). Finally, the blood, after having 

 undergone the action of the air, returns to the heart to com- 

 mence its course anew. 



Fig. 44. Circulatory Apparatus in a Mollusk.* 



111. Crustacea. In lobsters, crabs, and other animals 

 of this class, the blood follows the same course as in the mol- 

 lusca ; only the heart, destined to transmit the blood to all 

 parts of the body, consists of a single ventricle (Fig. 41), and 

 the veins are everywhere replaced by irregular cavities, which 

 have not the form of vessels, and which constitute, in the 

 neighbourhood of the branchiae, a sort of reservoirs, called 

 venous sinuses (Fig. 45). The venous blood thus bathes all 

 the organs ; but the nourishing fluid is once more collected 



* Anatomy of the Snail. a, the mouth ; b b, the foot ; c, the anus ; 

 d d, the lung ; e, the stomach, covered above by the salivary glands ; ff, in- 

 ine; a, the liver; h, the heart; i, aorta; j, gastric artery; I, hepatic 



artery ; k, artery of the foot ; m, abdominal cavity, supplying the place of 

 a venous sinus ; n n, irregular canal in communication with the abdominal 

 cavity, and carrying the blood to the lung ; 

 from the lung to the heart. 



3, vessel carrying the blood 



