BASIC ORGANIZATION OF THE METAZOA 85 



expanded like a flask, the gland is alveolar. Such an indented epithelium 

 may branch, that is, form subsidiary indentations {G, H), and then the 

 gland is termed compound, as contrasted with simple glands in which the 

 tube is not branched. Nearly all glands, in the higher animals at least, 

 have other tissues, including blood vessels, collected around or spread 

 among the epithelial part; but in every case it is the epithelium that 

 does the actual secreting. 



The Other Tissues. — The two types of tissues described in the 

 preceding sections are distinguished largely on structural grounds, while 

 the functions performed by different samples of them may be quite 

 unlike. The remaining tissues of those listed on page 81 are, however, 

 highly specialized for specific functions. They are so much more impor- 

 tant in connection with those functions than with respect to their struc- 

 ture that descriptions of them are deferred to later chapters. Contractile 

 tissue includes mainly the voluntary and involuntary muscles; nervous 

 tissue comprises all the nerve, brain, and ganglion cells; vascular tissue 

 includes the blood and the more fluid parts of the blood-producing organs 

 (red marrow, spleen); and reproductive tissue consists of the germ cells 

 and their forerunners. 



Organs and Systems. — An organ, generally speaking, is any structure 

 which performs a given function. In this general sense, a single cell 

 may be an organ, as in the case of single secreting cells scattered through 

 an epithelium. Usually, however, cells that do a certain thing are 

 grouped. Thus the secreting cells of Hydra which provide the adhesive 

 substance that holds the animal fast to other objects are all located on 

 the foot. Also, the stinging cells of Hydra show a tendency to be col- 

 lected in patches, particularly on the tentacles. Where such patches are 

 sharply marked off, as the glandular foot of Hydra, each group could be 

 considered an organ. 



Some biologists, however, reserve the term organ for a collection of 

 tissues acting together to perform some function. The stomach of a 

 vertebrate animal is a suitable example. The inner epithelium, just one 

 cell thick, does the secreting of the digestive fluid or fluids. Outside this 

 layer is a connective tissue layer rich in blood vessels and lymph spaces 

 by which the materials for secretion are brought in and the digested foods 

 are carried away. Covering this layer are two layers of muscles, running 

 in different directions and together serving to churn up the contents of 

 the stomach and mix them with the digestive fluids. The several tissues 

 are structurally unlike, but each contributes in some way to the digestion 

 of the food. The stomach is thus an organ in this more restricted sense. 



When a number of organs are occupied with different phases of a 

 complicated general process, they constitute a system of organs. The 

 mouth, esophagus, stomach, intestine, and several glands associated 



