Invertebrate Gallery of the Indian Museum. il 



on : much as happens in communities of men, where the 

 various industries and activities of the community are not 

 performed by all the citizens alike, but each citizen is 

 devoted to one particular occupation. In this way, since 

 '' practice makes perfect," each function comes to be dexter- 

 ously and economically performed. 



This apportioning of functions is known as the " physio- 

 logical division of labour," and we must now consider the 

 changes to which it gives rise. 



Let us imagine a colony of cells, all of which are alike 

 in form, and, like a Protozoan, possess the same limited 

 range of ill-developed functions, and let us now suppose 

 that to one group of cells is allotted the function of pro- 

 tecting the colony from external injuries, and to another 

 group of cells the function of digesting the food of the 

 colony. Then as a natural result of special perfection in 

 one set of protecting functions to the relative neglect of 

 the digestive functions, and vice versa, these two groups 

 of cells will gradually come to differ in many points of 

 structure and form. 



In other words, among cells originally alike in form 

 the specialization of function to which the division of 

 physiological labour gives rise leads to many differences 

 in form. 



A second consequence of the physiological division of 

 labour, and of the differences in occupation and struc- 

 ture to which it leads, is that all the various cells and 

 groups of cells — like the citizens of a state — come to be 

 mutually dependent on one another. 



The earlier and simpler stages of the physiological 

 division of labour are seen in the two large family-unions 

 or phyla of Metazoa known as Coelenterata, in which the 

 constituent cells of the body fall into two distinct well- 

 differentiated layers or membranes, — an external protec- 

 tive and sensory layer, and an internal digestive layer,— 

 enclosing a central cavity or stomach. 



