THE INFUSORIA 



379 



FIG. 26. 



cometes, and others which reach a considerable size there is never 

 more than one. 



In the majority of cases the contractile vacuole is spherical in 

 shape and situated in the medulla. It slowly 

 expands (diastole) until its periphery comes in 

 contact with the pellicle at the surface, when 

 it instantaneously collapses (systole). In some 

 forms (Spirostomum, etc.) a long canal may be seen, 

 towards the close of diastole, to be connected 

 with the spherical vacuole. This appears to be 

 formed by the fusion of a row of small secondary 

 vacuoles stretching from the anterior to the 

 posterior end of the body. In Stentor the con- 

 tractile vacuole has a very elongated, rod-shaped 

 form (Fig. 44). 



In the VORTICELLIDAE it is situated in the 

 neighbourhood of the vestibule, but instead of 

 opening directly into it when systole occurs, it 

 opens into a reservoir which is in communica- 

 tion with the vestibule. In Blepharocorys (Fig. 4) 

 and some other forms the contractile vacuole opens 

 into the passage which leads to the anus. In 

 Paranwecmm (Fig. 46, 3) and a great many other ^ ^act 

 forms a series of six or more canals or spindle- oiigotnchous infusorian 



. . T with several contrac- 



shaped secondary vacuoles appear in a radiating tile vacuoles. (After 

 form round the principal contractile vacuole, and * 

 may be seen to discharge their contents into the primary vacuole 

 immediately before its collapse. 



There can be little doubt that the function of the contractile 

 vacuole is primarily the excretion of waste products, but by assisting 

 the osmosis of fresh water through the protoplasm it may be 

 regarded as being also respiratory in function. As shown by 

 A. G. Bourne, when anilin blue is added to the water in which the 

 animal lives, the contents of the contractile vacuoles are deeply 

 stained. 



The systole of the contractile vacuole is not caused by any 

 active contraction on the part of the protoplasm of the medulla, 

 but, according to Btitschli, it is due to the physical attraction of 

 the small droplet of fluid to the mass of water at the periphery. 

 It is of the same general nature as the phenomenon of capillarity. 

 The action cannot take place until the vacuole has, by its diastole, 

 reached the periphery, and the drop of fluid is thereby brought 

 into contact with the surrounding medium. 



The rapidity of the successive contractions of the contractile 

 vacuoles is variable. According to Rossbach it always increases 

 with a rise in temperature. In Euplotes cJiaran the intervals 



