90 



THE VITAL FUNCTIONS. 



internal surfaces are very generally beset with ex- 

 tremely minute cilia, maintaining, by their vibra- 

 tions, continual currents of fluids in the cavities, 

 both of the body and of the tentacula.* 



If we considered the Medusa as having four sto- 

 machs, we might in like manner regard the Aste- 

 rias, or star-fish, as having ten, or even a greater 

 number. The mouth of this radiated animal is at 



the centre of the under surface ; it leads into a 

 capacious bag, situated immediately above it, and 

 w hich is properly the stomach. From this central 

 sac there proceed ten prolongations, or canals, 

 which occupy in pairs the centre of each ray, or 

 division of the body, and subdivide into numerous 

 minute ramifications. These canals, with their 

 branches, are exhibited at c, c, Fig. 258, which 

 represents one of the rays of the Asterias, laid open 

 from the upper side. The canals are supported in 

 their positions by membranes, connecting them 

 with the sides of the cavity in which they are sus- 

 pended. Numerous cilia are found spread over all 

 these internal surfaces. 



In the various species of JEchini, we find that the 

 alimentary tube has attained a greater develope- 

 ment; for instead of constituting merely a blind 

 pouch, it passes entirely through the body of the 



* Sharpcy, Cyclop, of Anat. and Physiol, art. Cilia, I. 614. 



