334 



ANIMAL DEVELOPMENT 



same house, each making boots on his own account, will corre- 

 spond to the more usual type of Protozoan colony. But the 

 workmen in a boot-factory, divided into sets, each set doing a 

 different part of the work, and all therefore interdependent, will 

 correspond to a Metazoon. Supposing the division of work to 

 be carried out imperfectly a lower Metazoon will be typified, but 



if carried out into detail 



J \ the nature of a higher 



Metazoon will be illus- 

 trated. 



Between the nu- 

 merous cells which 

 make up the body of 

 any Metazoon there 

 is always a sharp divi- 

 sion into (a) cells con- 

 cerned with develop- 

 ment from eggs, and 

 (b} ordinary somatic 

 cells which do the 

 rest of the work. We 

 may feel pretty sure 

 that this distinction 

 made its appearance 

 very early in the his- 

 tory of the primitive 



Fig. 858. Volvox. A, A colony, much enlarged; e, egg-cells; s, sperms IVIetaZOa. It IS 

 in various stages of development. B, Four individual members of colony in r , 



side view, still further enlarged, c, a sperm, greatly enlarged. VCry 



to know that there 



are some few colonial forms of Protozoa in which division of 

 labour has already begun to take place, and in which there is 

 propagation by means of eggs as well as by vegetative methods. 

 One such form is a little freshwater organism (Volvox, fig. 858), 

 which is about the size of a small pin's head, and of green colour. 

 The colony is a hollow sphere with a gelatinous wall, in which 

 are imbedded a large number of pear-shaped individuals the 

 bodies of which are connected together by threads of living sub- 

 stance, while from each of them two filaments {flagella) of similar 

 nature project to the exterior. The flagella execute lashing move- 

 ments, by means of which the colony swims, at the same time 



