78 MANUAL OF ANIMAL BIOLOGY 



3. Respiratory, Excretory, and Vascular Systems 



It has been emphasized repeatedly that the processes of respira- 

 tion and excretion must be continuously in operation in every liv- 

 ing cell. In a larger and more complex type of animal, like the 

 Starfish, it is necessary that arrangements be made so that the 

 oxygen necessary for respiration can be supplied to those cells of 

 the body which are not in contact with the surrounding water, 

 and in addition, so that the carbon dioxide and other metabolic 

 wastes can be removed. In other words a specialized transpor- 

 tation system is essential. This function is performed in the 

 Starfish by the coelomic fluid which flows through the tissues and 

 serves as a general transportation system for the materials passing 

 between the cells and their environment. Present in the coelomic 

 fluid are numerous isolated amoeboid cells, the amoebocytes, 

 which are formed from the tissue lining the coelom. 



In types of animals higher than the Starfish, such, for example, 

 as will be seen in the Earthworm, the much more complex vascular 

 system with the blood circulating through the body in closed 

 tubes, the blood vessels, performs essentially the same function. 

 It is the link between the cells and their environment. As a 

 matter of fact, in the Starfish, the possible forerunner of the 

 closed vascular system of the higher animals is to be found in 

 the so-called perihemal system, the tiny tubes of which run, for 

 the most part, parallel with and just below those of the water vas- 

 cular system. The perihemal tubes contain a small amount of 

 fluid which possibly is of the same nature as the blood of higher 

 animal types. 



With regard to the exchange of gases essential to respiration, the 

 pertinent facts may be stated as follows : The carbon dioxide 

 picked up by the coelomic fluid from the cells with which it 

 comes into contact is carried to the tubular dermal branchiae 

 which, as has been noted above, project through the body wall 

 into the surrounding water. The respiratory exchange is carried 

 on through the walls of the branchiae, the oxygen received passing 

 in turn to the tissue cells as the coelomic fluid is slowly moved 

 throughout the body, largely by ciliary action. 



In the excretion of nitrogenous wastes, it appears that the wastes 

 from the cells pass into the coelomic fluid, then are carried to the 

 rectal caeca through which they leave the body. The rectal 



