THE ARCHIPELAGO OF BREHAT. 107 



whilst in another case it can only act upon the mass 

 of the blood, which it is to vivify, by the inter- 

 vention of a single organ called a branchia or a lung, 

 according to its external or internal position.* "We 

 should find that the blood was either enclosed in 

 vessels f, or diffused over the entire body, bathing 

 every part of the organs which it was destined to 

 nourish.^ We should discover wants of every kind 

 giving rise to a multitude of instincts, and necessi- 

 tating a hundred differently varied forms of organic 

 apparatus, whilst each of the classes to which we 

 have already referred, would appear to be surrounded 

 by a system of groups, depending upon different 

 types, whose final representatives would merge 

 into one another, on the confines of these little 

 worlds. 



The Insects, Myriapods, Arachnidans and Crus- 



* Some Arachnidans respire by means of sacs enclosing lamellae, 

 which constitute the respiratory organs. This apparatus, which is 

 commonly designated by the term lungs, differs considerably from 

 the true lung of the Vertebrata. All the Crustacea, on the contrary, 

 including even those which live in the air, as the Oniscus, respire 

 by means of branchiae. 



| We find that Crustaceans, Arachnidans, &c., possess a heart and 

 a distinct vascular system. 



J In Insects the heart is prolonged into a dorsal vessel, and in 

 most cases the blood on issuing from this organ is immediately 

 diffused in the lacuna, or intercellular spaces. There is sometimes, 

 however, a rudimentary vascular system (Duges, Blanchard), but 

 in all the Articulata, properly so called, there is always a more or 

 less considerable interruption in the vascular circle, so that the 

 blood is finally poured into the lacunae. In the Crustaceans, for 

 example, the arterial apparatus is well developed, but there are no 

 veins. (See the Memoirs of MM. Audouin and Milne Edwards, 

 of which we have already spoken.) 



