DIFFERENTIATION OF STRUCTURE 27 



the food that enters this gastral or central cavity, and 

 the products of the digestion are absorbed by the cells 

 and distributed to all the cells of the body. As we 

 ascend the scale we find that this inner layer becomes 

 more and more specialised for this purpose, until we find 

 in the worms that the inner layer is nothing more or less 

 than an enzyme-producing layer, that is, a layer speci- 

 alised for digestive purposes. Further, this central layer 

 had in such forms as Hydra, a common opening for the 

 entrance of the food particles and the outlet of the 

 undigested remains, but as the enzyme specialisation 

 increased, a second opening, usually at the extreme 

 opposite end, was developed for the outlet of the un- 

 digested remains of the food, so that we have now 

 reached a stage, in the worm, for instance, where there 

 is a special tube in the body for the purpose of digesting 

 and dealing with the food. 



Once this tube has been acquired it does not dis- 

 appear again. It persists and continues to increase in 

 complexity. The anterior end or mouth now becomes 

 specialised for the purpose of breaking up the food 

 substances, and ultimately it develops such structures as 

 teeth for crushing the food. Differentiation continues, 

 and we find certain portions of the alimentary tube set 

 aside for special duties. A portion is set aside for the 

 carrying out of the digestive processes, and still another 

 for the absorption of the products of digestion. Again, 

 certain parts are set aside for supplying certain of the 

 digestive ferments, and this gradually leads to the 

 formation of special glands. The final result of this is 

 the highly complex alimentary canal we find in man 

 and in the higher vertebrates. 



