PHYSIOLOGY OF THE INVERTEBRATA. 183 



differentiated under the intliieuce of natural selection and the 

 struggle for existence. 



As already stated, in the lowest Invertebrates the digestive 

 and circulatory systems are not differentiated, but among 

 the higher Invertebrates these two systems become distinct. 

 The circulatory system only shapes itself after the digestive 

 system ; consequently one may look upon the former as an 

 appendage to the latter. 



In the higher animals the blood is made to pass through 

 the respiratory organs in order to expose it to the oxidizing 

 action of the air. In certain of the lower animals the air 

 penetrates into the body ; but in all the higher animals, and 

 in many of the lower, there exists a complex apparatus for 

 the circulation of the blood : (i) A system of blood-vessels to 

 convey the blood into the various parts of the body. (2) An 

 organ (called the heart) destined to put this fluid in motion. 

 ]\[ost animals, from man to the Aniielidii, have a heart. 



The Protozoa. 



In these creatures there is no true blood, yet there is a 

 curious foreshadowing of a circulation. In the RJiizopoda* 

 the only structures which may be said to have a circulatory 

 function are the contractile vacuoles. The spaces are filled 

 with a clear fluid, and exhibit fairly regular and rhythmic 

 expansion and contraction (diastole and systole). During 

 the systoles radiating canals or vessels extend from these 

 vacuoles ; these widen as the vacuole lessens in diameter. 

 Presently the vacuole begins to expand, whilst the radiating 

 canals become narrower in diameter and ultimately disappear. 

 The contractile vacuole performs more than one function, 

 and among these is probably that of circulation. There is a 

 pulsating central "organ" with conducting canals proceed- 

 ing therefrom. Does not this look very much like a primitive 

 circulatory system ? 



* The Uhizopoda includes the I'rotoplasta, Furui)iinifcr<i, and the liadic- 

 laria. 



