AND PHYSIOLOGY. 



243 



expand and contract regularly, but in a spiral, screw-like 

 manner, like the peristaltic motion of the intestines. 



434. Circulation in the Radiata and Protozoa. — In a 

 few polyps a vascular system is quite obvious, and may be 

 inferred in them all. The blood is colorless, and has white 

 corpuscles. In the Acalephse there exists a circulating fluid, 

 which is thought by Agassiz and Dana to be merely water, 

 intended to subserve the purposes of respiration rather than 

 nutrition. The Echinoderms have one heart, and sometimes 

 more. The Infusoria have pulsatory cavities, but no system 

 of circulation. 



433. How does the heart pulsate? 434. What is the plan of circulation among the 

 radiate animals ? 



