26 BIOLOGY 



the specialisation becomes more marked, there must be 

 some means of communication between the various cells, 

 and this may arise in a special modification of some or all 

 of the muscle-cells as a primitive type of nervous system. 



Further, as the muscular system increases, a skeleton 

 becomes necessary, if for nothing else, at least as a frame- 

 work for the attachment of the muscles. 



A better idea of the gradual increase in complexity, 

 and consequently in the gradual differentiation of cells 

 and the division of labour between the cells, may be 

 gained if we trace the development of a few of the more 

 important systems. 



All uni-cellular organisms nourish themselves without 

 any special organ being set aside for the preparation of 

 the food. In amoeba the food is digested anywhere, 

 and the nutritive substances are distributed throughout 

 the protoplasm, but in paramoecium the food particles 

 have a definite tract of circulation, and all follow it, so 

 that we may say there is a hint here of an alimentary 

 canal. In the colonial forms, such as Spirogyra or 

 Epistylis, there is nothing to suggest that any individual 

 contributes to the support of any other, but it is just 

 possible that this may occur in forms such as Micro- 

 gromia, as it certainly does in the case of Volvox. Among 

 the majority of plants a common interest with regard 

 to nutrition is soon established, and persists even among 

 the higher forms of plant life. 



In animals, on the other hand, we find that among 

 the lowest forms of multi-cellular organisms there is 

 a distinct set of cells set aside for nutritive purposes. 

 The layer of cells surrounding the central cavity in 

 Hydra produce some digestive ferment which acts on 



