CHAPTER VII. 



PROMORPHOLOGY. 



113. We have seen in the preceding chapters how the work which an 

 organism must do is divided among its parts, and that the parts become 

 specialized in connection with this division of labor. This complexity 

 which is known as organisation is, in any animal, the result of forces 

 both within and without the animal, and expresses the adaptation of the 

 internal structures to each other and to external conditions. The simplest 

 organism known is thus organized. Organization is merely more evident 

 in the more complex organism. In addition to the fact of the organiza- 

 tion and heterogeneity of structure it is easy to see, after examining a 

 number of animals, that these different parts are not thrown together 

 without some definite order. For example, the ordinary vertebrates move 

 with their long axis horizontal, and possess certain organs that we always 

 expect to see at the anterior end; their appendages are arranged in a 

 definite way in relation to the long axis. The parts of the starfish are 

 arranged according to a different but equally definite plan. All considera- 

 tion of the general plan according to which animals are constructed be- 

 longs to Promorphology. The fundamental plan may be similar in groups 

 of animals which are otherwise very different, because of similar external 

 conditions and similar modes of life. 



114. Definition of Sections. In trying to express the plan of struc- 

 ture in animals it is convenient to have in mind certain planes to which 

 we can refer the parts. A section perpendicular to the main axis of an 

 organism or of an organ is called a transverse or cross section. The 

 longitudinal median section separating the body into right and left halves 

 is a sagittal section. A longitudinal section, perpendicular to the sagittal 

 and separating the dorsal (or back) and ventral (or belly) portions of the 

 body is described as a frontal section. An animal is said to be sym- 

 metrical with regard to any of these planes when the parts severed by 

 the plane are essentially similar. 



115. Axiality. As an organism grows from its small beginnings in the 

 fertilized ovum, or from a spore in the simpler forms, the new materials 

 may be added more or less uniformly so that a mere increase in size 

 results ; or growth may take place more rapidly along some radii than 

 along others, making it depart from its original spherical form; or mate- 

 rials or organs of one kind may occur along one radius and different ones 

 on another (as in Fig. 46). These lines of special growth and develop- 

 ment are called axes. We may investigate them as to their number, their 

 space-relations to each other, and the likeness or unlikeness of the two 

 ends or poles of each axis. 



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