Ill COLONIES 281 



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 Vorticella or 

 Paramoecium. 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. 



The study of the foregoing Hving things and especially of 

 Bacteria (p. 152), the smallest and probably the simplest of all 

 known organisms, naturally leads us to the consideration 

 of one of the most important problems of biology — the 

 problem of the origin of life. 



In all the higher organisms we know that each individual 

 arises in some way or other from a pre-existing individual : 

 no one doubts that every bird now living arose by a process 

 of development from an egg formed in the body of a 

 parent bird, and that every tree now growing took its origin 

 either from a seed or from a bud produced by a parent plant. 



