292 THE WORLD OF THE SEA. 
appearance of one or two wheels rotating in opposite directions. 
The appearance is very deceptive, so that some naturalists have 
even described it as a permanent collar, continuously agitated by 
an undulating movement. This really violent movement for so 
small an animal often causes the rotifer to twist rapidly round on 
its own axis, as well-as glide through the water, propelled much 
after the fashion of a screw steamer. The vibratory cils are 
faithful to the function we have so often found them performing, 
inducing currents of water to enter the system of the animal, 
bearing the food and air necessary for its existence. This is 
BRACHIONS. 
economy indeed, to make» one organ perform several distinct 
offices. The same member which urges the animalcule through the 
drop of water, likewise causes it to eat and breathe. 
The rotifers vary, as all other classes, in the details of their 
construction, especially in their tails; some are without the caudal 
appendage, others rejoice in two ; some have long tails, some short. 
The tail is used, as in all other denizens of the watery world, as the 
helm of the ship; by its inclination this way or that the direction 
of the creature is determined. Some of these little creatures carry 
an external horn; of its use we can only conjecture, and the 
most reasonable supposition is, that it is for defence or perhaps 
attack. The mouth, which is very large, has the shape of a bell. 
It is provided with two lateral mandibles, which are horny tubes, 
furnished near the extremity with one or more teeth. The 
