54 ELEMENTARY ZOOLOGY 



kinds of cells. The beginnings of distinct tissues and 

 organs are indicated. 



Degrees in cell differentiation and body organization. 



In the study of the cellular constitution of the tissues 

 and organs of the toad, we found to what a high degree 

 the differentiation of the cells may attain, and in the study 

 of the anatomy of the toad we found how thoroughly these 

 differentiated cells may be combined and organized into 

 body-parts or organs. The body of the toad is made up 

 of distinct organs, each composed of highly differentiated 

 or specialized cells. The body of Hydra is composed of 

 cells for the most part only slightly differentiated and 

 hardly recognizably grouped or combined into organs. 

 These two conditions are the extremes in the body- 

 structure of the many-celled animals. Between them 

 is a host of intermediate conditions of cell differentia- 

 tion and body organization. When we come to the 

 study of other members of the great branch of simple 

 many-celled animals to which Hydra belongs (see 

 Chapter XVII), it will be found that some of them 

 show a slight advance in complexity beyond Hydra. 

 Higher in the scale of animal life the forms will be found 

 still more and more complex, with ever-increasing differ- 

 entiation of the cells, with the combination of the differ- 

 entiated cells into distinct organs, and the co-ordination 

 of organs into systems of organs up to the extreme shown 

 by the birds and mammals. And hand in hand with this 

 increasing complexity of structure goes ever-increasing 

 complexity or specialization of function. Breathing is a 

 simple function or process with Hydra, where each body- 

 cell takes up oxygen for itself, but it is a complex business 

 with the toad, or with a bird or mammal, where certain 

 complex structures, the lungs and accessory parts, and 

 the heart, blood-vessels and blood all work together to 

 distribute oxygen to all parts of the body. 



