CELL AGGREGATION AND DIVISION OF LABOR 95 



and covering. Furthermore there are cells of this layer which by means 

 of their thread-like stinging processes are able to serve as a means of 

 protection or assist in the killing of prey. As in the sponge no set of 

 cells is especially concerned with respiration, with the transfer of nutri- 

 tive solutions, or with the control of the responses of the organism, for 

 most or all of the cells share in these general functions. Thus in the 

 sponge and Hydra there are tissues but their number is small. 



Tissues, Organs, Systems. — In more highly organized metazoa 

 the tissues are often grouped into organs. A few examples of organs 

 will suffice to make clear the meaning of the term. In vertebrate animals 

 (those with l^ack-bones) the stomach is a highly developed organ. Its 

 inner lining is composed of a single layer of columnar epithelial cells 

 commonly called the mucosa. Portions of the mucosa are modified into 

 gastric glands (complex secretory structures) by which is secreted the 

 gastric juice which plays an important part in the digestion of proteins. 

 Surrounding this epithelial tissue is a layer of tough spongy tissue, called 

 connective tissue, which forms a good support for the delicate epithelial 

 cells. About the connective tissue are two layers of muscle cells, the 

 fibers of one layer running round the stomach and those of the other 

 extending lengthwise. They strengthen the stomach wall and by their 

 rhythmic contractions mix the contents of the stomach with the gastric 

 secretion. About the stomach is a layer of closely fitted cells called 

 the peritoneum which acts as a fine, tough, well-lubricated covering. To- 

 gether these tissues make up the stomach. Such an organization of tis- 

 sues into a structure which as a whole performs certain functions is called 

 an organ. Other organs are the heart, liver, kidneys, brain, eye, tongue, 

 etc. 



Only a part of the general function of digestion is carried on by the 

 stomach. Other organs which play important parts in the process of 

 digestion are the small intestine, pancreas, liver and large intestine. These 

 organs together with the organs for food taking and for the voiding of 

 indigestible portions of the food comprise the alimentary system. The 

 term system may be defined as an aggregation of organs which cooperate 

 in the performance of one or more general functions. Very complex 

 animals have several systems, among which are the respiratory, circu- 

 latory, excretory, genital, muscular, sustentative, and nervous systems. 



The number and complexity of the systems possessed by an animal 

 are to a certain extent an index of the degree of its organization. The 

 lowest and simplest metazoa, as Hydra and the sponge, are made up of 

 tissues and it is scarcely proper to use the term organ in referring to any 

 of their structures. Higher metazoa, contrariwise, have many of their 

 tissues incorporated into organs which may be grouped into systems. 

 Higher animals usually have more systems than lower ones and their 

 systems are more complex and the functioning of the systems more com- 



