INFUSORIA. 63 



members of tlie group. The mouth is often sur- 

 rounded with cilia. These cilia, as we have seen 

 in the case of Vorticella, are usually continued 

 into the oesophagus, though the latter would seem 

 to be in some cases destitute of these appendages. 

 In most Infusotia the oesophagus presents the 

 appearance of an open tube, freely hanging down 

 into the cavity of the body; but in some of these 

 animals it is completely collapsed, and it is only in 

 Vorticella and a few of its allies that it has been 

 observed to widen below into a pharjrnx. Eecently, 

 however, it has been proved by the observations of 

 Lieberkiihn, that in Trachelius and Loxodes the 

 oesophagus is continued into a peculiar ramified 

 canal. In other Infusoria it is altogether wanting, 

 and in these the alimentary apparatus consists 

 merely of a mouth leading into a cavity excavated 

 through the parenchyma of the body. In Ghilo- 

 don and Nassula, the interior of the oesophagus is 

 provided with a number of peculiar rod-like "teeth'' 

 arranged in the form of a cylinder (fig' 13, h). 

 Besides the oral orifice, many Infusoria are pro- 

 vided with an anus, which in Steoitor, Vorticella, 

 and certain of their allies, is situated not far from 

 the mouth, close beneath the surface of the disk, 

 whilst in others, e. g. Biirsaria, it is placed at the 

 posterior extremity of the body. 



7. Contractile vesicle. — We have already 

 noticed in A'tnoeba and ActinopJwys the existence 

 of certain clear spaces which occur in the substance 

 of the body, and in which movements of con- 

 traction and dilatation have been seen to take 

 place. Similar contractile vesicles have been ob- 

 served in most of the true Infusoria, being usually 

 situated in some part of the parenchyma of the body 



