18 ON CLASSIFICATION. 



of an elongated oval shape (Fig. 4, A B d), which has been 

 called the " nucleus," though it must be carefully distinguished 

 from the " nucleus " of a cell. Upon one side of this, and, as it 

 Avere, stuck on to it, is a little rounded body (Fig. 4, B d'), 

 which has received the name of the " nucleolus." The animal 

 swims about, driven bv the vibration of its cilia, and whatever 

 nutriment may be floating in the water is appropriated by 

 means of the current which is caused to set continually into the 

 short gullet by the cilia which line that tube. 



But it is a singular circumstance that these animals have an 

 alimentary canal consisting of a mere gullet, open at the bottom, 

 and leading into no stomach or intestine, but opening directly 

 into the soft central mass of sarcode. The nutritious matters 

 passing down the gullet, and then into the central more fluid sub- 

 stance, become surrounded by spheroids of clear liquid (Fig. 4, 

 Ad), consisting apparently of the water swallowed with them, so 

 that a well-fed Paramoecium exhibits a number of cavities, each 

 containing a little mass of nutritious particles. Hence formerly 

 arose the notion that these animals possess a number of stomachs. 

 It was not unnaturally imagined that each of the cavities in 

 question was a distinct stomach ; but it has since been dis- 

 covered that the outer layer of the sarcode is, by means of some 

 unknown mechanism, kept in a state of constant rotation ; so 

 that the supposed stomachs may be seen to undergo a regular 

 circulation up one side of the body and down the other. And 

 this circumstance, if there were no other arguments on the same 

 side, is sufficient to negative the supposition that the food- 

 containing spaces are stomachs ; for it is impossible to imagine 

 any kind of anatomical arrangement which shall permit true 

 dilatations of an alimentary canal to rotate in any such manner. 

 Fsecal matters are extruded from an anus, which is situated not 

 far from the mouth, but is invisible when not in use. It is an 

 interesting and important character of the Infusoria, in general, 

 that, under some circumstances, they become quiescent and 

 throw out a structureless cyst around their bodies. The Infu- 

 sorium then not unfrequently divides and subdivides, and, the 

 cyst bursting, gives rise to a number of separate Infusoria. 



The remarkable powers of multiplication by fission and 



