XII DIMORPHISM • 135 



fission were extended downwards so as to include the distal 

 end of the stalk, the result would be a branched, tree-like 

 stem with a Vorticella-like body at the end of every branch. 



As a matter of fact, this process takes place not in Vorti- 

 cella itself, but in a nearly allied infusor, the beautiful 

 Zoothamnium, a common genus found mostly in sea-water 

 attached to weeds and other objects. 



Zoothamnium arbuscula (Fig. 26, a) consists of a main 

 stem attached by its proximal end and giving off at its distal 

 end several branches, on each of which numerous shortly- 

 stalked bell-animalcules are borne, like foxgloves or Canter- 

 bury-bells on their stem. The entire tree is about i cm. 

 high, and so can be easily seen by the naked eye : it is shown 

 of the natural size in Fig. 26, b. 



We see, then, that Zoothamnium differs from all our 

 previous types in being a compound organism. 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. 



As in Vorticella, the stem consists of a cuticular sheath 

 with an axial muscle-fibre {ax.f), which, at the distal end 

 of the main stem, branches, like the stem itself, a prolonga- 

 tion of it being traceable to each zooid (d). So that the 

 muscular system is common to the whole colony, and any 

 shock causes a general contraction, the tree-like structure 

 assuming an almost globular form (c). 



It will be noticed from the figure that all the zooids of 

 the colony are not alike : the majority are bell-shaped and 

 resemble Vorticellae (a, n. z, and d), but here and there are 

 found larger bodies (a, r. z, and e) of a globular form, with- 

 out mouth, peristome, or disc, and with a basal circlet of 

 cilia. The characteristic band-like nucleus (««) and the 



