342 



EESPIRATION IN LOWER ANBIALS 



every cell, and thus each cell can effect its exchange of 

 gases directly, in the same manner as the single cell of the 

 protozoan. 



The starfish and its allies. — In the starfish we have the 

 simplest kind of a respiratory system. In these animals 

 a tube is found which conveys water to various parts of 



Fig. 154 Vertical section of one arm of fi stariish ; b, ampulla ; d, stone canal ; 



i, radial arabulacral vessel ; ft, feet. — Davison Zoology. 



the body. The same tube is used in distributing water 

 to the tube feet (see p. 305), and this tube serves not only 

 to make locomotion possible, but also, by the water which 

 it carries, to distribute oxygen to various parts of the star- 

 fish body and to receive carbon dioxide from them. These 

 starfishes also have soft membranelike extensions of the 

 skin that line the body cavity and project as pouches, or 

 pockets, between the plates on the upper surface of the 

 body. Oxygen is absorbed from the sea water into the 

 body cavity through these thin, membranous pockets, 

 and carbon dioxide passes out through them, also. The 

 starfish, therefore, has two entirely distinct methods of 

 respiration. It is to be noted that while this animal also 

 possesses a partly developed circulatory system the oxy- 

 gen distribution is not dependent upon it. This water- 

 tube system of distribution is also entirely distinct from the 



