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 (Opalina), although Vejdovsky 

 ('92) describes contractile vacuoles in a closely 

 allied form, Monodontoplirya 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 

 Hoplitophrya (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 Urocentntm. 

 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 

 other runs around the peristome edge. Fabre- 

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

 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 

 Opkrydlum eichhornii Ehr. 

 [Wrzesniowsky.] 



c, the reservoir of the vacu- 

 ole (v) emptying through a 

 long canal into the vestibule ; 

 0, the oesophagus ; n, the nu- 

 cleus. 



