Ill VORTICELLA 273 



acquired the large number of nuclei characteristic of the 

 genus. 



Here, then, we have an interesting case of progressive 

 differentiation or development (p. 9) ; the organism begins 

 life as a very small uninucleate mass of protoplasm, and as it 

 increases in size increases also in complexity by the repeated 

 binary fission of its nucleus. 



The next organism we have to consider is a ciliated infusor 

 even commoner than those just described. It is hardly 

 possible to examine the water of a pond with any care with- 

 out finding in it, sometimes attached to weeds, sometimes 

 to the legs of water-fleas, sometimes to the sticks and stones 

 of the bottom, numbers of exquisitely beautiful little creatures, 

 each like an inverted bell with a very long handle, or a wine- 

 glass with a very long stem. These are the well-known 

 " bell-animalcules ; " the commonest among them belong to 

 various species of th^ .genus Vorticella. 



The first thing that strikes one about Vorticella (Fig. 72, a) 

 is the fact that it is permanently fixed, like a plant, the 

 proximal or near end of the stalk being always firmly fixed 

 to some aquatic object, while to the distal or far end the 

 body proper of the animalcule is attached. 



But in spite of its peculiar form it presents certain very 

 obvious points of resemblance to Paramoecium and Opalina. 

 The protoplasm is divided into cortex (c, cort) and medulla 

 {med), and is invested with a delicate cuticle («). There is 

 a single contractile vacuole {c. vac) the movements of which 

 are very readily made out owing to the ease with which the 

 attached organism is kept under observation. There is a 

 meganucleus {nu) remarkable for its elongated band-like 

 form, and having in its neighbourhood a small rounded 

 micronucleus. Cilia are also present, but the way in which 



Phact. Zool. T 



