CHAPTER VII 



PROMORPHOLOGY 



117. 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 con- 

 nection with this division of labor. This complexity which is known as organiza- 

 tion 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 organization 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, most 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 consideration of the general plan according to which animals are con- 

 structed may be called Fromorphology. 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. 



n8. Definition of Sections. — In trying to express the plan of structure 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 symmetrical with regard to any of these planes when the parts severed by the 

 plane are essentially similar. 



119. 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 the organism depart 

 from its original spherical form; or materials or organs of one kind may occur along 

 one radius and different ones on another (as in Fig. 48). These lines of special 

 growth and development 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. 



120. Types of Symmetry. — It is desirable to distinguish the following types: 

 I. In a spherical organism in which no differentiation is apparent (as in a 



simple spherical cell, or blastula, Pigs. 13 A, 3; 46) any plane passing through 

 the centre divides it into symmetrical parts and all the axes or diameters, are 



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