ANNULOSA: ROTIFERA. 249 



masticatory or digestive apparatus, and more or less closely 

 resembling the young form of the species. The most charac- 

 teristic organ in the great majority of the Rotifera, is the so- 

 called " wheel-organ," or " trochal disc," which is always situ- 

 ated 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 im- 

 pression 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 some other forms (Gastrotricha), there is 

 no true wheel-organ, capable of protrusion and retraction, but 

 the cilia are variously disposed over the surface of the body. 

 The Chcetonoti or Hairy-backed Animalcules have no jaws, 

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

 They have often been placed in the Turbellaria, but there 

 seem to be good reasons for regarding them as an aberrant 

 group of Rotatoria. Balatro and Apsilus are non-ciliated in 

 the adult condition. 



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. 122, 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 

 of this apparatus are horny, and are believed by Mr Gosse to 

 be homologous with the parts of the mouth in insects. In the 

 females of almost all known species of Rotifera the intestinal 

 canal is a more or less simple tube, extending through a well- 

 developed perivisceral cavity, and terminating posteriorly in a 

 dilatation, or " cloaca," which forms the common outlet for the 

 digestive, generative, and water-vascular systems. 



In both sexes there is a well-developed water-vascular sys- 

 tem, usually consisting of the following 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." 



