230 BIOLOGY. [Book ii. 



respiratory part. A mammif er grows cold and dies when its skia 

 is covered with an impermeable varnish.^ If in a mammifer we 

 keep by means of a receiver air in contact with the skin, this air 

 is modified as absolutely as in the lungs. 



But, as we have previously seen, the fine capillary vessels are 

 of all the parts of the body of complex animals the best organised 

 for the exchanges, whatever they may be, between the interior 

 medium and the exterior medium. Consequently an organic 

 surface in contact with the atmosphere breathes the more 

 freely and energetically the better provided it is with capillary 

 vessels and the more immediate the contact of these vessels is 

 with the exterior air. (Fig. 34). Rich capillary vascular- 

 isation, easy contact with the aerian gases; such are the two 

 principal conditions of a good respiratory surface. And these 

 conditions can be met with apart from the respiratory organs 

 properly so called. It is thus that the pond leech (Colitis posdlis) 

 breathes in part through the intestine. In this animal, the 

 intestinal muco'us membrane being excessively vascular, almost 

 entirely constituted by sang^neous capillaries connected by a 

 small quantity of conjunctive substance, the animal makes use 

 of this substance as of a respiratory organ ; he swallows the air, 

 which, in contact with the intestinal capillaries, is exchanged 

 for the carbonic acid which the intestine expulses.^ 



Nevertheless, every animal in any degree complex has points 

 of his exterior or interior surface specially charged with the 

 respiratory gaseous exchange. It follows, as a matter of course, 

 that for the respiratory system, as for all others, specialisation 

 is the more perfect, the higher the animal is exalted in the 

 hierarchy. Before passing in review, in this respect, the diverse 

 groups of the animal kingdom, it is useful to define some terms 

 often recurring in the description. 



The name of hranchiae is usually given to all the appendices, 



1 FotircaiUt, Influence des Enduits Impermeables, etc. [Comptes Smdus de 

 I'Academie des Sciences, t. XVI.) 

 ^ Leydig, Traite d! Histologic, etc., p. 417. 



