144 NATURAL HISTORY 



GENUS XVI. 



VORTICELLA. 



This is an extensive genus. MUUer enumerates seventy- 

 five species, to which Bruguiere has added four others ; 

 but I am inclined to think many of these are only different 

 stages of the creature's development. They vary much 

 in their organization; hence I have formed them 

 into two divisions. They are naked, contractile, and 

 possess cilia circularly disposed near the mouth, pro- 

 ducing a vortex in the water : in many of them they 

 appear to rotate like a wheel ; to account for which, va- 

 rious hypotheses have been invented. According to 

 Dr. E. this is occasioned by their peculiar disposition, 

 and not from any dilTerence of structure; for as w^ith the 

 vibrating cilia, the base of each is a bulb (similar to the 

 spines of the Echina), which by means of muscular 

 fibres it can move in any direction, like a ball and 

 socket joint. Thus each cilium in its revolution de- 

 scribes a cone, the apex of which is the bulb. Now, if 

 the cilia are arranged in a circle, and viewed laterally 

 while in motion, the whole wheel will appear to revolve 

 as each cilium passes ; first, a little nearer the eye on this 

 side of the cone, and then further off on the other side. 



