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MANUAL OF ZOOLOGY. 



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 intes- 

 tinal 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. 



Fig. 51. Rotifera. A Diagrammatic representation of Hydatina senta (generalised 

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

 Mouth ; c Pharyngeal bulb or mastax, with the masticatory apparatus ; d Stomach ; 

 e Cloaca; ./Contractile bladder ; gg Respiratory or water-vascular tubes ; h Nerve- 

 ganglion giving filament to ciliated pit (k] ; o Ovary. B Melicerta ringens. (After 

 Gosse.) 



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

 tem, usually consisting of the following parts : In the hinder 

 part of the body, close to the cloaca, and opening into it, is a 

 sac or vesicle, which is termed the " contractile bladder," and 

 exhibits rhythmical contractions and dilatations. From the 

 contractile bladder proceed two tubes " the respiratory tubes" 

 which pass forwards along the sides of the body, and ter- 

 minate anteriorly in a manner not quite ascertained. Attached 

 to the sides of the respiratory tubes, in all the larger Rotifera, 

 is a series of ovate or pyriform vesicles, each of which is fur- 

 nished internally with a single central cilium, which is fixed to 

 the free end of the vesicle. It is asserted, however, that these 

 ciliated vesicles communicate internally with the perivisceral 

 cavity with its contained corpusculated fluid. The exact func- 



