ANNULOIDA : ROTIFERA. 



155 



and so marked with apparent intelligence their movements, so 

 various their forms and types of structure,' that they form one 

 of the most interesting departments of zoological and micro- 

 scopical study. They are all 

 aquatic in their habits, and 

 in the great majority of cases 

 are free-swimming animals, 

 some, however, being per- 

 manently fixed, as is the 

 case with Steplianoceros - , 

 (fiq. 43) and Floscularia. *"^i 



s 



They are usually simple, but 

 are occasionally composite, 

 forming colonies, as inMega- 

 lotrocha. As a rule, the male 

 and female Rotifera differ 

 greatly from one another, 

 the males being smaller 

 than the females, destitute 

 of any masticatory or diges- 

 tive apparatus, and more or 

 less closely resembling the 

 young form of the species. 

 The most characteristic or- 

 gan in the great majority of 

 the Boti/era is the so-called 

 'wheel-organ,' or 'trochal 

 disc,' which is always situ- 

 ated at the cephalic or dis- 

 tal end of the body, and 

 consists of a circlet of cilia, 

 which, when in action, vi- 

 brate so rapidly as to pro- 

 duce the illusory impression 

 that the entire disc is rota- 

 ting. The disc, which car- 

 ries the cilia, is capable of 

 eversion and inversion, and 

 may be circular, reniform, 

 bilobed, four-lobed, or di- 

 vided into several lobes. It 



Serves the purpose of loco- 



motion in the free-swim- 



ming forms, and in all it 



serves to produce currents in the water, which convey the food 



to the mouth. 



Fig. 43. Rotifera. The Crown Animalcule 



gf ^' * faed E M " 



