I80 MANUAL OF ZOOLOGY. 



of which has been a good deal disputed, and is still doubtful. 

 They are looked upon here as a distinct division of the Scale- 

 cida, following Huxley; but they are very frequently placed- 

 with the Annelida amongst the lower division of the Annulosa 

 (Anarthropoda). 



The Rotifera are Annuloida of a minute size, never parasitic, 

 inhabiting water, and usually provided with an anterior ciliated 

 disc, capable of inversion and eversion. In the females there is 

 a distinct mouth, intestinal canal, and anus. A nervous system 

 is also prisent, consisting of ganglia, situated near the anterior 

 extremity of the body, and sending filaments backwards. A water- 

 vascular system is also present. 



Most of the Rotifera are entirely invisible to the naked 

 eye, and they are all extremely minute, none of them attain- 

 ing a greater length than i-36th of an inch. Nevertheless, 

 as remarked by Mr Gosse, " so elegant are their outlines, 

 so brilliantly translucent their texture, so complex and yet so 

 patent their organisation, so curious their locomotive wheels, so 

 unique their apparatus for mastication, so graceful, so vigorous, 

 so fleet, and so marked with apparent intelligence their move- 

 ments, so various their forms and types of structure," that tliey 

 form one of the most interesting departments of zoological and 

 microscopical study. They are all aquatic in their habits, and 

 in the great majority of cases are free-swimming animals, some, 

 however, being permanently fixed, as is the case with Stephano- 

 ceros, Melicerta (fig. 53, B), and Floscularia. They are usually 

 simple, but are occasionally composite, forming colonies, as in 

 Megalotrocha. As a nde, the male and female Rotifera differ 

 greatly from one another, the males being smaller than the 

 females, destitute of any masticatory or digestive apparatus, 

 and more or less closely resembling the young form of the 

 species. The most characteristic organ in the great majority 

 of the Rotifera is the so-called " wheel-organ," or " trochal 

 disc," which is always situated at the cephalic or distal end 

 of the body, and consists of a retractile disc surrounded by a 

 circlet of cilia, which, when in action, vibrate so rapidly as to 

 produce the illusory impression that the entire disc is rotating. 

 The disc, which carries the cilia, is capable of eversion and 

 inversion, and may be circular, reniform, bilobed, four-lobed, 

 or divided into several lobes. It serves the purpose of loco- 

 motion in the free-swimming forms, acting somewhat like the 

 propeller of a screw-steamer, and in all it serves to produce 

 currents in the water, which convey the food to the mouth. 



In Chcetonotus, and one or two other' forms, there is no true 

 wheel-organ, capable of protrusion and retraction, but the cilia 



