270 RAMBLES OF A NATURALIST. 



sarily fill the general cavity of the body. Here 

 it bathes the greater part of the viscera, and receives 

 without any intermediate aid the nutrient elements 

 which have been elaborated by the alimentary canal ; 

 hence it will be understood that if we must admit 

 the presence of venous and arterial blood in the 

 higher Molluscs, we are yet unable to distinguish 

 either lymph or chyle. 



The Articulata furnish precisely similar facts. 

 Several of the results which have been obtained 

 from these animals had already long before been 

 9,dmitted into the science of zoology ; but in con- 

 sequence of not perceiving the relations which 

 connect them with what occurs in other groups, 

 physiologists had been led to regard them merely as 

 strange and characteristic exceptions. Thus, from 

 the time when the researches of MM. Audouin and 

 Milne Edwards received the prize of the French 

 Institute, in 1827, the absence of veins where the 

 heart and arterial system were both present, was 

 looked upon as exclusively characteristic of lobsters, 

 crabs, and other animals belonging to the class of the 

 Crustacea. The absence of any circulatory organ 

 was, they thought, limited to Insects and to a portion 

 of the Arachnlda, and they endeavoured to explain 

 this fact, which was the more striking from its pe- 

 culiar isolation, by the modification which the re- 

 spiratory apparatus presents in these cases. 



Insects, indeed, have neither lungs nor branchiae ; 

 but in them the air passes through a variable number 

 of openings into a system of tubes, called tracheae, 

 whose singular structure bears the most striking 



