i84 PHYSIOLOGY OF THE INVERTEBRATA. 



" In the Infnxorla, contractile vacuoles are present, and 

 there is also a curious movement of the outer layer of the 

 sarcode in company with the food vacuoles. It will be 

 remembered that these food vacuoles pass, after quitting the 

 abrupt termination of the oesophagus, through the sarcode 

 along a very definite line. They trace the outline of the in- 

 fusorial body as they pass along just within the contractile 

 layer of the animal. With them the outer layer of the sar- 

 code is said to move." 



The Porifera. 



In the Porifera or S}wngida, there is no true blood, but 

 there is a circulation of water carrying food particles and 

 air for respiration. This circulation is brought about by the 

 action of cilia, which cause the currents of water to enter the 

 inhalent pores, and after traversing the internal canals, finally 

 take their exit through the exhalent pores. These currents 

 of wa,ter containing nutritive matter act as carriers of tissue- 

 forming materials as well as of waste products, consequently 

 we may regard them as representing the circulatory system 

 among these Invertebrates. Altliough the water currents in 

 the Porifera have a circulatory function, they also perform 

 the functions of respiration and digestion. 



The Ccelenterata. 



In these animals the blood or nutritive fluid is not con- 

 tained in any vessels, but is free in the somatic cavity or 

 enteroccele. This fluid is moved by " the contractions of the 

 body, and, generally, by cilia developed on the endodermal 

 lining of the enteroccele." By this means a kind of circula- 

 tion is constantly maintained. The movements of the body 

 of the animals belonging to the Caikntcrata cause a move- 

 ment of the corpusculated blood in the body cavities, a flux 

 and reflux, a flowing and an ebbing of the nutritive fluid. 

 Here is the most general form of circulation. There are no 



