PHYSIOLOGY OF THE INVERTEBRATA. 223 



The Tkichoscolices. 



The water-vascular systems, which are often ciliated, con 

 stitute internal respiratory organs. The water which per- 

 meates these tubules contains oxygen. Respiration in many 

 of the organisms belonging to this class is also performed by 

 the external surface of the body ; and no doubt internal or 

 tissue respiration takes place by means of various respiratory 

 pigments. 



The Annelida. 



In the Annelida the principal seat of respiration is the 

 pseudo-haemal system. For instance, in both the Hirudinea 

 and Oligochceta this system of vessels is well developed, and 

 has been alluded to in a previous chapter. 



The blood which these vessels contain consists of a coloured 

 plasma, and it is stated to possess no nutritive properties. 

 The pigment present in this fluid is hemoglobin.* 



In reality the fluid contained in these vessels has a respira- 

 tory function and contains air in solution. The pseudo- 

 haemal systems are in communication with the external 

 medium. This communication of the respiratory system 

 with the air or water in which the animal lives probably 

 serves the purpose of the gaseous interchanges which are so 

 essential in animal life. There must be a constant accumu- 

 lation of carbonic anhydride in the respiratory fluid, and 

 therefore a constant diminution of the quantity of oxygen. 

 The oxygen must be restored, the carbonic anhydride excreted. 

 Both these important ends are attained by means of this 

 ever-recurring communication between the water-vascular 

 system and its homologues on the one hand, and the animal's 

 environment on the other. Hence in the lowest as well as in 

 the highest forms of the animal kingdom, cutaneous respira- 

 tion is an important adjunct to the various organs or devices 



* MacMunn has proved that the integument of Lumbrkus terrestris con- 

 tains haematoporphyrin {Journ. of Physiol., vol. 7, p. 248). 



