290 THE PROTOZOA 



force, surface tension. The liquid of the vacuole mixes quickly 

 with the surrounding water, as a small drop fuses with a larger mass, 

 as soon as the intervening layer of protoplasm gives way before the 

 pressure of the growing vacuole. Delage and Herouard ('96), on 

 the other hand, held that contractility of the protoplasm brings about 

 the contraction just as it causes the expulsion of undigested food 

 matters. 



When present, carbon dioxid is probably dissolved in the water of 

 the contractile vacuole, although, in some cases, especially among the 

 Sarcodina, gas vacuoles containing, probably, carbon dioxid have been 

 repeatedly observed, first by Perty ('49) in Arcella, and subsequently 

 by Biitschli ('74), Engelmann ('78), Entz ('78), and others, not only 

 in Arcella, but in other rhizopods as well. According to Engelmann, 

 the gas is secreted very rapidly, but at irregular intervals, and he, with 

 other modern observers, accepted and confirmed the suggestion made 

 by Perty, that the gas vacuoles serve as an hydrostatic apparatus, by 

 means of which the organisms can raise and lower themselves in the 

 water. 



Although the contractile vacuole appears to play an important role 

 in respiration, it is not absolutely necessary for the performance of 

 this function, for a great many forms have no such organ. The ma- 

 rine rhizopods and Radiolaria, and some of the fresh-water forms of 

 Rhizopoda (e.g. Pelomyxa\ have no contractile vacuoles. Respira- 

 tion in such cases must take place by osmosis. An interesting series 

 of forms are the Opalinidae, parasitic ciliates of which some genera 

 have a contractile vacuole with numerous feeding canals (Anoplopkrya, 

 Hoplitophrya), while others have no contractile vacuole, although the 

 canals are present (Opalina, according to Fabre-Dumergue). In 

 none of these forms is there a mouth, and respiration must take place 

 by osmosis. 



There is, on the whole, very little direct evidence to support the 

 conclusion which on a priori grounds appears indisputable, that, like 

 other organisms, the Protozoa take in oxygen and give off carbon 

 dioxid. The fragmentary evidence which we have tends to the con- 

 clusion that, when present, the contractile vacuole, probably in addi- 

 tion to other excretory functions, is the active agent in the disposal of 

 carbon dioxid, while the income of oxygen-holding water takes place 

 by osmosis through the body walls, by ingulfing through the mouth, 

 or by both methods. 



C. SECRETION AND EXCRETION 



Carbon dioxid is but one of the waste matters formed by the decom- 

 position of proteids in vital activities. In the higher animals the final 



