CHAPTER XIII. 



CIRCULATION. 



1. General HorpJiology of the Circulation. 



IF we embrace with a complete and comprehensive glance the 

 anatomy of the circulatory apparatus in the whole animal king- 

 dom, we still see, as in other cases, specialisation gradually 

 effected. Nature, as it was formerly the fashion to say. did not 

 arrive at perfection at a bound. She made numberless attempts, 

 long she groped, adding successively new pieces to the system, or 

 complicating little by little those which existed already. In 

 modern language, the circulatory apparatus, like all others, was 

 perfected and differentiated more and more under the influence 

 of the struggle to live, and of natural selection. 



A first fact results from comparative anatomy, namely, that 



the circulatory system only shapes itself in living organisms, 



after the digestive system, of which at the outset it may be 



^viewed as the appendix. 



In the protozoa l there is not yet either digestion or circulation. 

 The nutritive liquids absorbed pass direct into the nearly homo- 

 geneous parenchyma of the body. The liquids expulsed come 

 forth in the same fashion by a sort of insensible perspiration. 



Jn the rhizopods the intimate contact of the substances absorbed 



with the living substance is aided by the sarcodic contractions, by 



1 Gegeubaur, Anatomie Comparte, p. 103. 



