ANNULOIDA : ROTIFERA. 



199 



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 Chatonotus, 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 

 Chatonoti or Hairy-backed Animalcules have no jaws, and have 

 the ventral surface of the body clothed with cilia. They have 

 usually been placed in the Turbellaria, but there seem to be 

 good reasons for regarding them as an aberrant group of Rota- 

 toria. 



Fig. 73. Rotifera A, Diagrammatic representation of Hydatina sent a (generalised 

 trom Pritchard). a Depression in the ciliated disc leading to the digestive canal ; b 

 Mouth : c Pharyngealbulb or mastax, with the masticatory apparatus ; d Stomach ; 

 e Cloaca : /"Contractile bladder : g g Respiratory or water-vascular tubes ; h Nerve- 

 ganglion giving filament to ciliated pit (Jt) ; o Ovary. B, Melicerta ringens. (After 

 Gosse.) 



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. 73, 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- 



