GASTEROPODA. 



409 



these two canals. Now as tlieir communication with the abdomi- 

 nal cavity is evident and palpable, whether we call them vena 

 cava, or cavities analogous to a right ventricle, or branchial 

 arteries, for it is evident that they fulfil the functions of these 

 three organs, the inevitable conclusion is, that fluids poured into 

 the abdominal cavity can become directly mixed with the mass of 

 the blood and thus conveyed to the branchiae, and that the veins 

 perform the office of absorbent vessels. 



This extensive communication is undoubtedly a first step to- 

 wards the establishment of that, still more complete, which nature 

 has established in insects, where, as we have seen, there are not 

 even distinct vessels of any kind appointed for taking up the nutri- 

 tive fluid. From these facts Cuvier concludes that no proper 

 absorbent system exists in the Mollusca, still less in animals 

 inferior to them in the scale of creation. 



The vein ^.189. 



appointed to 

 convey the re- 

 novated blood 

 from the bran- 

 chiae to the 

 heart, when slit 

 open (./zg.189, 

 d), exhibits the 

 orifices of the 

 smaller vessels 

 derived from 

 the respiratory 

 laminae arrang- 

 ed in circles. The auricle of the heart is made up of reticulated 

 fibres (e), and when laid open it is seen to be separated from 

 the more muscular ventricle (g) by a valve (/), whereby any 

 retrograde movement of the blood is prevented. 



(444.) Such is the construction of the heart in a great majority 

 of the GASTEROPODA ; but in a few of the lowest orders, namely, 

 those most nearly allied to the CONCHIFERA, slight modifications 

 are met with. Thus in Chiton (Jig. 197), so remarkable from 

 the singularity of its shelly covering, the heart is situated in the 

 middle of the posterior region of the back, and is furnished with 

 two auricles, one appropriated to each lateral series of branchiae ; 



