742 COMPARATIVE PHYSIOLOGY. 



ficance; it carries away waste products, both solid and 

 gaseous, and removes these from the body. 



The fluids of Ascidians, Lamellibranchs, and of a few 

 Gasteropods, are all classed by Krukenberg as hydro- 

 lymph. They consist largely of water, but contain in 

 addition formed elements, or dissolved proteids. In 

 Ascidians, the body fluid contains a small amount of dis- 

 solved proteids, and some pigmented corpuscles. Its real 

 function has not been fully investigated ; the presence of 

 dissolved proteids seems to suggest digestion by ferments 

 in spite of Kruken berg's negative results. 



In Echinoderms we find that both a perivisceral fluid 

 and blood enclosed in special blood vessels are present. 

 Of the blood little or nothing is known, the technical 

 diiBculties in the way of isolation being very great. The 

 perivisceral fluid contains numerous formed elements, and 

 a small amount of dissolved proteids. It probably performs 

 the functions of the lymph of Vertebrates, but is said to 

 have a respiratory function in addition. 



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 

 Cephalopods, the blood is both respiratory and nutritive. 

 It is "hajmolymph," 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 



