WHEEL-ANIMALC ULES. 



)i the cilia, by producing currents in the water all converging 

 towards the mouth, insure an abundant supply of food, by hurry- 

 ing down the gaping throat whatever minute aliment may be 

 brought within range of the vortex thus caused ; or, on the other 

 hand, if the animal disengages itself from the substance to which 

 it is held by its curious anchor, the wheels, acting upon the prin- 

 ciple of paddle-wheels, carry it rapidly along with an equable 

 gliding movement. 



T, 2. Brachionns. 



FlG. 177. ROTIFERA. 



3. Sleplianoceros. 



Some of these little creatures are enclosed in a transparent 

 shell, often variously armed with spines at one or both extremi- 

 ties, but others are not so enclosed. 



The eggs of the Rotifera form beautiful objects for micro- 

 scopic study. They are covered with a transparent shell, through 

 which the parts of the embyro, as they develope themselves, 

 gradually become distinctly apparent, until at length the cilia 

 are seen performing their mimic rotation, though as yet the im- 

 prisoning shell has not been broken. At last, by the action of 

 these organs, which every moment becomes more energetic, the 

 transparent membrane is ruptured, and the little creature bursts 

 forth, eager to enter upon its new existence, and already possess- 

 ing the form of its parent. The time from the exclusion of the 

 egg to the hatching is commonly about twelve hours. Ehrenberg 

 watched an individual through eighteen successive days : it was 

 full grown when he first observed it, and it did not die of old age 

 at last. Such an individual he found to be capable of producing 

 four eggs every twenty-four hours, the progeny derived from which 

 grow to maturity and exclude their fertile ova in the same period, 



