REVOLVING ANIMALS. 15 



is very transparent; so that while it serves as a 

 common shelter within which each individual can 

 retreat at will at the approach of danger, it does 

 not obstruct the view of the beholder. When kept 

 for any time beyond an hour or two under observa- 

 tion in a contracted space and in a warm atmosphere, 

 this gelatinous envelope becomes soft; and the 

 individuals forming the colony, no longer held 

 closely and firmly together, slip out one by one, 

 stray about helplessly, and soon perish. This 

 probably is the reason why these creatures flourish 

 best in cold weather. In some instances the 

 mucous investment of the Conochilus is green, or 

 partially so, and is probably caused by the nature 

 of the food on which they have been feeding ; and 

 this beautiful gauzy transparent green membrane- 

 ous matter enveloping the white crystalline ro- 

 tating colony, adds greatly to its beauty. 



Before this strange little colony breaks up, 

 let us note it more particularly. Remark that 

 each member is furnished with an apparently re- 

 volving or rotating wheel-like head in fact, a 

 wreath of cilia which, lashing the water from nearly 

 all points at once, produces a whirl or vortex. The 



