536 



GASTEROPODA. 



the mass of the blood and thus conveyed to the branchiae, and that the 

 veins perform the office of absorbent vessels. 



(1418.) This extensive communication is undoubtedly a first step 

 towards 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 nutritive 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. 



(1419.) The vein appointed to convey the renovated blood from the 

 branchiae to the heart, when slit open (fig. 270, d), exhibits the orifices 



Fig. 270. 



Vena cava of Aplysia laid open. 



of the smaller vessels derived from the respiratory lamina arranged 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 (y) by a valve (/), whereby any retrograde movement of the 

 blood is prevented. 



(1420.) In Aplysia, the arterial blood, having been distributed 

 throughout the body by means of the heart and aortic vessels, is received 

 into a capillary system, which forms a rich network composed of minute 

 vessels, the walls of which are perfectly distinct ; but these capillaries 

 are found not to be continuous with any system of recurrent vessels, 

 but gradually resolve themselves into little lacuna) formed amongst the 

 interstices which occur between the bands of cellular membrane and 

 the fibres of various tissues. These vacuoles communicate in their turn 

 with larger lacunae, situated beneath the common integuments, or occu- 

 pying the interspaces between the muscular fasciculi of the foot of the 

 mantle, and of other parts of the body. The result of this arrangement 

 is the formation of a vast system of venous cavities, dispersed through- 

 out the abdominal parietcs. In the foot and in the lobes of the mantle 



