TUBE -DWELLING ROTIFERS. 29 



former gets a mansion built with mathematical 

 precision, on a symmetrical plan, from materials 

 carefully selected, the latter gets only a mud -hovel 

 formed, or thrown loosely together. While the 

 one structure is strong and enduring, forming a 

 monument of stability even when uninhabited, the 

 other is always falling " about the ears," so to 

 speak, of its possessor. 



Between these two extremes are several others, 

 of varying perfection in structure and capacity. 



Our illustration shows a group of Limnias 

 ceratophylli, the "two-lipped tube-wheel of the 

 Hornwort." The Ceratophyllum demersum or Horn- 

 wort, as it is commonly called is its favourite re- 

 sort, and has originated the specific name of the 

 creature ; but it is by no means specific of its 

 habitat, for the animal will attach itself to almost 

 any water-plant, and is found in great abundance 

 in the canals about London. 



Somewhat resembling the Melicertans, Limnias 

 yet presents many differences, the most noticeable 

 being the form of the flower-like wheel and the 

 structure of the tube-like dwelling. But while M. 

 ringens builds with bricks, and M. pilula with pills 



