748 COMPARATIVE PHYSIOLOGY. 



In Insects the blood is of the nature of Vertebrate lymph. 

 It is very rich in dissolved proteids, and undoubtedly serves 

 for the nutrition of the tissues. It has no respiratory 

 function, in spite of the frequent occurrence of various 

 pigments in it — a point of some theoretical interest. The 

 tracheal tubes carry air, and so oxygen, to every part of the 

 body; an oxygen-carrying fluid formed by the organism 

 itself thus becomes quite unnecessary. We may, physio- 

 logically, compare the tracheal system of the Insect with 

 the canal system of the Medusa. In both cases the ex- 

 ternal medium is carried by special channels to the tissues 

 themselves ; in both cases the body fluids have, in conse- 

 quence, no respiratory significance. 



In " Worms," Crustaceans, most Gasteropods, and Cepha- 

 lopods, the blood is both respiratory and nutritive. It is 

 " hsemolympb," combining the functions of the blood and 

 the lymph in Vertebrates. 



In Annelid worms the blood contains small formed 

 elements, and a number of respiratory pigments, some of 

 which will be discussed later. 



In Cephalopods the blood contains formed elements 

 similar to leucocytes, while in the plasma a respiratory 

 pigment known as haemocyanin is dissolved. This con- 

 sists of a proteid substance united to copper, and is the only • 

 albuminoid present in the plasma. It is very widely spread 

 among Gasteropods, Crustaceans, etc., but is not universal. 

 Its absence in some crabs, which have apparently no com- 

 pensating metal-containing pigment, perhaps indicates that 

 too much stress should not be laid upon its respiratory 

 significance. Lipochrome pigments are very frequently 

 present in the blood of Crustaceans and Cephalopods ; their 

 use is unknown. 



If we compare the condition seen in Cephalopods with 

 that found in Vertebrates, we find that in the latter it is the 

 red blood corpuscles which are the oxygen-carriers, while in 

 the former the plasma alone subserves respiration. Even in 

 Vertebrates, however, the waste carbonic acid is carried in 

 the plasma in combination with its soda, so that the plasma 

 is not entirely unconcerned with respiration. In both 

 Vertebrates and Cephalopods the plasma has a nutritive 

 function. 



