ANNULOIDA: ROTIFERA. 



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 mast;ication, so graceful, so vigorous, 

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

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

 fonn 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. 51, B), and Floscularia. They are usually 

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

 Megalotrocha. 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 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 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 locomotion 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 

 are variously disposed over the surface of the body. 



The proximal extremity of the body in the free forms ter- 

 minates in a caudal process, or " foot," sometimes telescopic, 

 which ends in a suctorial disc, or in a pair of diverging " toes," 

 which act as a pair of forceps (fig. 51, A). 



The mouth usually opens into a pharynx, or "buccal funnel," 

 which is generally provided with a muscular coat, constituting 

 the " mastax " or " pharyngeal bulb/' and which generally con- 

 tains a very complicated masticatory apparatus.* The parts 



* The lower jaws, or "incus," consist of a fixed portion, the "fulcrum," 

 to which are attached two movable blades the " rami. " The upper jaws, 

 or " mallei," consist each of a handle, or " manubrium," to which is hinged 

 a toothed blade, or "uncus." 



