MECHANISM OF THE CIECULATION. 65 



blood from the respiratory apparatus (d), which this liquid 

 reaches by venous canals more or less complete (n). Thjs is the 

 case in snails, oysters, and all acephala, as well as the class 

 gasteropoda ; but sometimes there exist no auricles, and a kind 

 of venous hearts is found altogether distinct from the aortic 

 ventricle, and situated at the base of the respiratory organs ; 

 this takes place in the sepia, and other cephalopoda. How- 

 ever it may be, the arterial blood in all these animals tra- 

 verses the heart, then proceeds to all the parts of the body, 

 and is afterwards directed towards the respiratory organs. 

 But in this latter part of its course, the blood is not always 

 contained in vessels properly so called. Sometimes the veins 

 are altogether wanting, their place being supplied by lacunae, 

 or void spaces filling up the intervals of the organs ; at other 

 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. 53). Finally, the blood, after having 

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

 mence its course anew. 



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. 50), 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. 54). The venous blood thus bathes all 

 the organs ; but the nourishing fluid is once more collected 

 into vessels, whence it proceeds from the gills to the heart. 

 The circulation is, consequently, semi-vascular and semi- 

 lacunar. 



112. Insects. In insects the blood is no longer con- 

 tained in any particular system of vessels ; there are neither 

 arteries nor veins, and the nourishing fluid is spread about 

 in the interstices of the organs ; still the circulation, such as 

 it is, is animated by the action of a vessel called dorsal, situ- 

 ated in the mesial plane of the body above the digestive tube 

 (Fig. 55). We shall consider, further on, the route followed 

 by the blood in the organism of those animals with a lacunar 

 circulatory apparatus. 



113. Worms. In the worms of the class annelides (such 



