92 ELEMENTS OF BIOLOGY 



to exaggerate the property o£ irritability and represent a very simple 

 type o£ nerve tissue. Still others are armed with special defensive 

 organs, not found in any other group of animals except the relatives 

 o£ hydra. Between the inner and outer layers is a binding layer of 

 jelly-like material that is formed by the metabolism of both the 

 inner and outer cell layers. It may contain a few detached cells 

 derived from the inner layer, but is never itself a continuous sheet 

 of cells. Some of these cells between the inner and outer layers are 

 destined to become sex cells. Those which become the male or 

 sperm cells divide into a large number, forming a pouch by distend- 

 ing the outer layer; this structure is known as the spermary. A 

 somewhat similar distention, the ovary, is caused by the increase in 

 size of other cells as they develop into mature female or tgg cells. 

 Both sperms and eggs are shed into the water, where fertilization 

 takes place. The animal also reproduces by budding, the bud as it 

 becomes larger and more mature separating from the parent. 



A review of the way in which the different types of cells in hydra 

 contribute to the life processes of the whole animal, when contrasted 

 with the comparatively simple division of labor among the cells of a 

 volvox colony, emphasizes the very much greater complexity of the 

 organization of the metazoon body, even in the most simple form. 

 Larger and still more complex Metazoa contain literally millions 

 of cells among which the functions of life of the animal as a whole 

 are divided. But division of labor does not mean independence; 

 each tissue operates in harmony with all other tissues, the whole 

 being coordinated to form an individual, a unity. 



Before leaving the subject of unicellular animals, it is well to 

 point out in advance of the presentation of facts concerning the 

 structure and relations of cells in Metazoa, that metazoon cells 

 have many characters in common with Protozoa, not only as re- 

 gards their physiological processes of energy transformation and 

 disposal of the wastes of metabolism, but also with regard to struc- 

 ture. For example, cells in the lining of the human respiratory sys- 



