in COLONIES 277 



As a matter of fact, this process takes place not 

 in Vorticella itself, but in some nearly allied infusors, 

 such as Carchesium and Epistylis. Each of these forms 

 consists of a main stem attached by its proximal end 

 and giving off at its distal end several branches, on 

 each of which numerous bell-animalcules with short 

 stalks are borne, like foxgloves or Canterbury bells on 

 their stem. 



We see, then, that Carchesium and Epistylis differ 

 from all our previous types in being compound organisms. 

 The entire " tree " is called a colony or stock, and each 

 separate bell-animalcule borne thereon is an individual 

 or zooid, morphologically equivalent to a single Vorti- 

 cella or Paramoecium. The colony is therefore an 

 individual of a higher grade than the zooid ; and such a 

 multicellular animal as a frog, the cells of which differ 

 markedly in structure and function, is an individual of 

 a higher grade still. 



As in Vorticella, the stem of Carchesium consists of a 

 cuticular sheath with an axial muscle-fibre which, at the 

 distal end of the main stem, branches like the stem itself, 

 a prolongation of it being traceable to each zooid ; so 

 that the muscular system is common to the whole colony, 

 and any shock causes a general contraction of all the 

 zooids. The stalk of Epistylis, on the other hand, is 

 non-contractile. 



PRACTICAL DIRECTIONS 



Paramoecium. Spread a little cotton-wool on a slide 

 over a drop of water containing Paramcecia (see note on 

 p. 238), in order to entangle them in its meshes, and put 

 on a cover-glass. Examine first with the low power and 

 then with the high power. Note 



i. The elongated form of the animal ; its anterior (more 

 rounded) and posterior (more pointed) end ; its flattened 

 dorsal and ventral surfaces ; and its month, near the middle 

 of the ventral surface. 



