250 



MANUAL OF ZOOLOGY. 



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 



Fig. 122. Rotifera. A, Diagrammatic representation of Hydatina senta (generalised 

 from Pntchard) : 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; f Contractile bladder; gg Respiratory or water-vascular tubes ; h Nerve- 

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

 Gosse.) 



the sides of the body, and terminate anteriorly in a manner not quite ascer- 

 tained. 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 furnished 

 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 function of this water-vascular system is not known, but 

 it is most probably respiratory and excretory. Dr Leydig believes that 

 water enters the perivisceral cavity by endosmose, where it mingles with 

 the absorbed products of digestion, to form the so-called ' ' chylaqueous 

 fluid ; " and that the effete fluid is excreted by the respiratory tubes, and 

 ultimately discharged into the cloaca by the contractile bladder. Taking 

 this view of the subject, Mr Gosse believes that the "respiratory tubes re- 

 present the kidneys, and that the bladder is a true urinary bladder ; " and 

 consequently that the ' ' respiratory and urinary functions are in the closest 

 relation with one another." This observer, further, finds a decided analogy 

 between the above system in the Rotifera and the long and tortuous renal 

 tubes of the Insecta, to which class he believes the Rotifera to be most 

 nearly allied. 



No central organ of the circulation, or heart, and no organs 

 of respiration are present, but the perivisceral cavity is filled 

 with a corpusculated fluid. 



