in COLONIES 277 



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 Vorticella or 

 Paramcecium. The colony is therefore an individual of a 

 higher grade than the zooid, and such a multicellular animal 

 as a frog 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. 



ParamCBCillHl. 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 



1. 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. 



2. The active movements, due to the cilia covering the body. 



3. The marked distinction between cortex and medulla. 



4. The characters of the elastic cortex : (a} the superficial cuticle, 

 and deeper striated layer ; (b] the cilia arising from the deeper layer, 

 and projecting through the cuticle ; (c] the trichocysts small oval sacs, 

 imbedded in the deeper layer ; (d) the two spherical contractile vacuoles. 



