THE INFUSORIA 



I8 7 



absent) by absorption through the body wall, and it is the function of 



the contractile vacuole to get rid of the surplus. This organ is 



variously complicated by the development of a more or less extensive 



series of canals, which empty in a common excretory vacuole. 



Always situated in the cortical plasm, the con- 



tractile vacuoles are fixed in position and com- 



municate with the exterior at systole by a 



permanent aperture, which, however, becomes 



covered internally during filling or diastole. 



They vary in number from one to a hundred, 



or even more, and are absent, apparently, in 



only one form (Ofalina), although Vejdovsky 



('92) describes contractile vacuoles in a closely 



allied form, Monodontophrya longissima, while 



even in Opalina the reminiscence of the vacu- 



ole is seen in the remnants of the feeding 



canals (Delage, '96). In its simplest form the 



vacuole is single and terminal, a condition 



which may be found in each of the four orders. 



When there are more than one, they are 



grouped around the original vacuole in a ter- 



minal position, or arranged along one or more 



lines upon the dorsal side. In Discophrya and 



//tf//z/0//^77#(Holotrichida) there is no regular 



vesicle, but a long contracting canal which 



runs the length of the body. Spirostomum 



(Heterotrichida) has a terminal vesicle, with 



one long feeding canal, and from this the 



canal system is developed in a variety of ways. 



Thus there is a vesicle with two feeding canals 



in Climacostomum (Fig. 91, B), one terminal 



vesicle and four feeding canals in Urocentrnm. 



In Stentor there is a single vesicle near the 



peristome, with two feeding canals, one of 



which runs to the end of the body, while the ole () emptying through a 



,, i i i T> i l n l canal into the vestibule ; 



other runs around the peristome edge. Fabre- Oi the oesophagus ; , the nu- 

 Dumergue ('89) holds that canals, for the most cieus. 

 part invisible, are present in all ciliates. This 



is certainly true in Frontonia, where there are one or two vesicles on 

 one side and an immense number of feeding canals, which anasto- 

 mose and branch to form a complicated network, involving the entire 

 body. In some forms the vesicle communicates with the exterior 

 directly, but it may be complicated by the formation of ducts or 

 reservoirs. In the holotrichous form, Lembadion, the vesicle lies 



Fig. 102. Anterior end ot 

 Ehn 



c , the reservoir of the vacu- 



