112 



ELEMENTARY ZOOLOGY 



muscles in the walls of the ampullae the fluid in the cavity 

 is compressed, thereby forcing the tube-feet out. By the 

 contraction of muscles in the tube-feet they are again 

 shortened while the small disk-like terminal sucker clings 

 to some firm object. In this way the animal pulls itself 



calcareous spine respiratory caeca 



epithelium of the body 

 cavity ^ 



mesentery- 

 pyloric caecumr- 



lacral ossicle 

 ectodermal covering* 



-ossicles 



ampulla 



pedicellaria U / / ^tube fool 

 .radial ''canal / 



radial blood-vessel 



KIG. 19. Semi-diagrammatic figure of cross-section of the ray of a starfish, 



Asterias sp. 



along by successive " steps." This entire system, called 

 the water-vascular system, is characteristic of the branch 

 Echinodermata. In addition to the fluid in the water- 

 vascular system there is yet another body-fluid, the peri- 

 visceral fluid, which bathes all of the tissues and fills the 

 body-cavity. 



TECHNICAL NOTE. Take a drop of the perivisceral fluid from a 

 living starfish and examine under high power of microscope, noting 

 the amoeboid cells it contains. 



The perivisceral fluid is aerated through outpocketings 

 oi the thin body-wall which extend outward between the 

 calcareous plates of the body. These outpocketings have 



