vii evolution of organisation 323 



. The Evolution of Organisation in Multicellular 



Animals 



The Evolution of the Germinal Layers as Primitive Organs 



Multicellular animals must have arisen from colonies, 

 formed in consequence of the advantage of association, of uni- 

 cellular forms still at a low grade of development, of protozoa 

 which had yet developed no kind of permanent organs except, 

 perhaps, the nucleus. The way in which these colonies were 

 formed was, probably, that the new protozoa produced by 

 division did not separate, but remained connected until the 

 colony had a sufficient number of members. But a solid 

 sphere (Morula) could not have been the most suitable 

 arrangement of those members, although they may at first 

 have assumed that arrangement, as some do still ; the best 

 arrangement must have been that which enabled all the 

 members to exercise their powers for the benefit of the whole 

 and the parts, and as these powers were the same in each, 

 this condition could only be fulfilled when all the cells were 

 equally exposed at the surface, so that they formed a single 

 layer constituting the wall of a spherical vesicle containing a 

 central cavity. Thus all the individuals were in the besf 

 position to obtain equal information as to the conditions ot 

 the outer world, and for common defence, as in our square of 

 infantry bristling with bayonets. There are also living forms 

 still showing this arrangement of the " blastula," e.g. the co- 

 lonial Volvocidse, consisting of unicellular flagellate Algse, 

 each of which directs its flagellum outwards, as the soldier his 

 bayonet. Before long, to effect locomotion in a definite direc- 

 tion, certain more powerful cells must have gained a supe- 

 riority over the rest and assumed the leadership, so that now the 

 several cells co-ordinated the movements of their processes. 



