Recent Researches on the Ififusoria. 303 



continues narrow till near the anus, where it is dilated into a sort 

 of cloacal enlargement, being surrounded through most of its 

 length by a cellular apparatus, composed of a number of small 

 recesses or pouches, which probably serve for absorption (Fig. 6) ; 

 they are not stomachs, for stomachs receive the food immediately 

 after it is swallowed, whereas these recesses are never filled in 

 the first instance. This form is found in the naked Zygotrocha, 

 the Rotifer^ Actinurus, Philodina, and others. 



These differences in the alimentary canal of the Rotatoria are 

 so constant and so well defined, that they might afford characters 

 for subdividing the class, and the four principal divisions thence 

 resulting might be designated, from the peculiarities of their 

 alimentary canal, by the terms, 1. Trachelogastrica, 2. Ccelogas- 

 trica, 3. Gasterodela, and, 4. Trachelocystica. Such a mode of 

 arrangement is, however, objectionable on two grounds, first, 

 because, as a general rule in arranging organized bodies, it is 

 preferable fo take the characters of the more subordinate divi- 

 sions from external and not from internal parts ; and, secondly, 

 because, in adopting the form of the alimentary canal as a 

 principal ground of subdivision, animals are brought together 

 which differ widely from each other in their general structure 

 and habit. The above is therefore given merely as a physiolo- 

 gical view of the alimentary organs, and is not brought forward 

 as a principle of arrangement. 



The Polygastric infusoria might also be divided into four 

 groups, distinguished by the peculiar form of their alimentary 

 organs, and designated accordingly, namely, 1. Anentera, 2. 

 Cycloccela, 3. Orthoccela, 4. Campy locoela (Fig. 7-10)* In 

 the Anentera the several sacs or stomachs, the plurality of which 

 characterizes the class, are unconnected by a canal, they all open 

 by a common orifice or mouth, and there is no anus (Fig. 7). 

 In the Cyclocwla, the alimentary canal, into which the stomachs 

 open, forms a circle or loop, having both an entrance and an 

 exit, or a mouth and anus, but these open externally by a com- 

 mon orifice (Fig. 8). The union of the mouth and anus into a 

 common orifice at the anterior part of the body, forms an exter- 

 nal character by which the Cychcaela may be always distin- 

 guished. The Orthocwla have a straight alimentary canal, with 

 a mouth and anus placed at opposite ends of the body (Fig. 9). 



