92 ORGANISMS OF TISSUES 



worms and annelids it becomes a long tube with mouth at one 

 end and anus at the other. After this the chief advance is in 

 the concentration of different cell types and specialization of 

 the tube into receptacles and digestive glands. The mouth 

 becomes an organ with jaws and teeth for masticating food, and 

 with salivary glands to moisten it; through a gullet the food 

 passes to a single or multiple digestive viscus, the stomach, 

 and thence through a long intestine where the products of 

 digestion are usually absorbed into the blood vascular system. 

 Intra-cellular digestion ceases entirely; the cells instead are 

 differentiated into various kinds of ferment producers whose 

 secretions are poured into the digestive tract and combine to 

 make all kinds of food, proteins, fats, and carbohydrates, 

 ready for absorption through the walls of the intestine. Gradu- 

 ally the digestive organs acquire a close relation with the 

 excretory organs, so that useless or damaging products of diges- 

 tion may be removed in the liver from the loaded blood before 

 distribution to the general system. In this way an organ sys- 

 tem is slowly evolved from a primitive digestion apparatus like 

 that of Hydra with its protozoa-like peculiarities. Similarly 

 with other organ systems, specialization and continued division 

 of labor result in the complex aggregates of cells, tissues and 

 organs which we know in the higher animal types. 



B. EXCRETION AND RESPIRATION. Practically all cells of 

 Hydra are in contact with the surrounding water, the ectoder- 

 mal cells directly with the surrounding medium, the endodermal 

 system with water taken in with food. As with protozoa, no 

 special organs are necessary for excretion and respiration but 

 the waste matters of metabolism are given off directly by 

 transfusion from the cells, and oxygen is absorbed by the 

 protoplasm from the water. In respiration this is exactly the 

 manner in which higher animals get their oxygen; the only 

 difference is that the cells of hydra absorb it from the medium 

 water while the cells of higher animals absorb it from the me- 

 dium blood or the medium air (insects and other tracheates). 



C. IRRITABILITY Hydra reacts to external stimuli of all 

 kinds by contraction of the muscle processes of the epithelial 



