INTRODUCTION. 3 



mals are so numerous and various in their bearings, and in- 

 vestigators approach the subject from such different points of 

 view that it is necessary, in order to express these results, to 

 divide zoology into several branches or departments. It must 

 be held in mind, however, that these divisions are more or less 

 artificial, and that the facts of each department are to be con- 

 sidered in connection with those of all the others, if they are 

 really to be understood. With all its departments, animal life 

 is to be thought of as a whole. Structures exist for the per- 

 formance of function, and the activities are intended to adjust 

 the animal to its whole life-relation. 



6. Morphology is the branch of the science which deals 

 with form or structure in its broadest sense, whether internal 

 or external, partial or total. In its most general sense it em- 

 braces the study of animals from the standpoint of symmetry, 

 that is, the form of the organism with reference to certain 

 planes passing through the body. For example, the human 

 body may be so divided by a single plane that two essentially 

 similar parts result, the right and the left. Again similar 

 parts may succeed each other in a linear series, as in the seg- 

 ments of the earth-worm ; or they may radiate from a central 

 point, as in the arms of the star-fish. This is the most funda- 

 mental kind of morphology. It relates the organism to space. 

 It is called Promorphology, and is related to Zoology some- 

 what as the study of crystals is to Mineralogy. 



Anatomy is that department of morphology which treats 

 of the structure of parts of the individual, as the organs and 

 systems of organs, the tissues, the cells, and so forth. This is 

 known as gross anatomy if the study pertains to the larger 

 units, as organs ; it is called Histology, if the constituent ele- 

 ments of these organs (as tissues and cells) are to be con- 

 sidered. 



Thus far we have thought of structure as stationary or per- 

 manent. As a matter of fact we know that each organism 

 begins life in a very modest way, as a single " cell," and grows 

 more complex by fairly well-defined stages until the adult con- 



