THE PRESIDENT'S ADDRESS. 469 



digestive cavity. This process actually takes place in a variety 

 of ways in the development of different animals at the present 

 day, and we need not stop to inquire which of these ways 

 represents most closely the course originally followed in ancestral 

 history. 



We have now arrived at a very definite type of multicellular 

 structure the gastrula of embryologists with a well-marked 

 differentiation of the constituent cells into two very distinct 

 groups with widely different functions an outer protective layer 

 and an inner layer concerned in nutrition, known in embryonic 

 forms as epiblast and hypoblast, in adults as ectoderm and 

 endoderm respectively. 



An organism possessing this type of structure has passed 

 definitely beyond the stage of a mere colony of Protozoa, and 

 constitutes what we may term an individual of the second order. 

 The best and most familiar example of such an individual is the 

 freshwater polype Hydra, which differs from an embryonic 

 gastrula in little more than the budding out of tentacles around 

 the mouth and a certain amount of histological differentiation 

 amongst the constituent cells of both ectoderm and endoderm. 



The organism has now gained a fresh starting-point for further 

 evolution ; there is a new unit of a higher order with which 

 to build, and it is extremely interesting to see how the next 

 really great advance begins, just as it did amongst the Protozoa, 

 with colony formation. 



Almost the only type of colony met with amongst the 

 Coelenterata, however, is the branching type, but the mode of 

 branching is extremely various. No great advance has been 

 attained in this way, though some of the colonies produced are of 

 much interest in discussing the problem of individuality. This 

 is especially true of the Siphonophora, those freely floating colonies 

 of Hydrozoa which form such an important constituent of the 

 oceanic plankton. In many of these we find differentiation 

 and division of labour amongst the constituent individuals 

 carried to such a high degree, and accompanied by so complete 

 an integration, that one is tempted to regard them as something 

 more than mere colonies, for in such integrated colonies the con- 

 stituent individuals tend to become converted into mere organs 

 subserving the welfare of the whole and quite incapable of 

 independent existence. 



