General Plan of Organization. 25 



man. On the other hand, in ordinary description little advantage is 

 to be gained from adherence to this principle. The terms anterior 

 and posterior apply with greater force to a lower vertebrate than to man, 

 while the terms superior and inferior are only of interest in the latter. 

 In this case the rule here followed is to use the terms anterior and pos- 

 terior for descriptive purposes without reference to the human relation. 

 The same applies to the terms of direction — upward, downward, forward, 

 and backward. It may be pointed out, however, that it has become 

 the practice with some to employ in place of anterior and posterior, such 

 terms as cephalic and caudal, thereby eliminating one of the difficulties; 

 or similarly to use the termination — ad, in connection with words sig- 

 nifying position, for the purpose of indicating direction {e.g., dorsad, 

 cephalad, laterad, for dorsalward, etc.). 



Reference may also be made here to the fact that the human 

 structures to which identifying names are applied frequently fail in one 

 way or another to correspond to structures in a lower form. Composite 

 structures to which individualizing names are applied, for example, may 

 be represented by independent parts. Also, structures which are similar 

 in form or function may be convergent. Finally, although it is essen- 

 tial to endeavor to apply all terms as accurately as possible, it will be 

 remembered that a terminology primarily arranged for one type cannot 

 be exactly applied to another without considerable qualification. 



THE GENERAL FEATURES AND GROUND PLAN 

 OF THE ORGAN SYSTEMS. 



In order to arrive at a proper understanding of the special features 

 of organs, or of the relations to one another of organ groups, it will be 

 found useful in many cases to consider them in the light of their deriva- 

 tion from a general or ground form. A brief statement of what may 

 be accomplished in this way and of the methods involved has already 

 been given above (p. 4) in discussing the general interpretation of 

 structure and the zoological position of the type, so that in the following 

 pages only the actual facts of organization will be considered. In this 

 connection it will be remembered that the subject is a very broad one, 

 and in the present case it will not be possible to do more than select 

 in the various organ systems the more important features, the con- 

 sideration of which will be of direct assistance in the practical study of 

 the type. Before taking up the individual organ systems, however, 

 it will be advisable to refer in the first place to the plan of classification, 

 and secondly to explain the more fundamental features of vertebrate 

 organization as something underlying the disposition of the systems 

 themselves. It will also be convenient to summarize the chief features 

 of the embryonic organ systems, since it is by reference to embryonic 

 conditions that the general features of the body are most readily recog- 

 nized. 



