CHAPTER 7 

 BASIC ORGANIZATION OF THE METAZOA 



Beyond the evolutionary stages traced in the last chapter, ending 

 with the differentiation of the somatic cells into a number of kinds, 

 the metazoa have gone various ways in great groups. Within each group 

 there is much in common, both in structure and in physiology; but 

 between groups there are many differences. Scarcely anything is com- 

 mon to them all. There are a few features, however, that are character- 

 istic of several or many of the great groups. Some of the more important 

 of these frequent structural conditions should be passed in review. 



Symmetry. — Symmetry is an arrangement of parts in relation to 

 planes, straight lines or points. A point is a position in space; it has 

 no dimension or size. A straight line is the shortest distance between 

 two points; it has only one dimension, length. A plane is, in common 

 words, a flat surface; more precisely it is a geometric figure of two dimen- 

 sions — length and breadth but no thickness — such that if any two 

 points in it be connected by a straight line that line is everywhere within 

 the figure. 



Symmetry is defined as a correspondence in shape or arrangement of 

 parts on opposite sides of a dividing line or plane, such that if the portion 

 on one side were viewed in a mirror it would appear identical with the 

 part on the other side. A symmetrical surface is divided into the corre- 

 sponding parts by a straight line; solid (three-dimensional) objects, 

 including animals, are divided into their equivalent parts by a plane. 

 The plane which divides a body into its corresponding halves is called the 

 plane of symmetry. Objects have difi'erent types of symmetry (Fig. 64) 

 depending on the number of planes of symmetry which may be passed 

 through them. If only one such plane is possible, the symmetry is 

 bilateral. Most animals (including all the higher ones) are bilaterally 

 symmetrical. They possess anterior and posterior ends which differ, 

 right and left sides which are alike except for the reversed order, and a 

 dorsal side (at or toward the back) and a ventral side (literally per- 

 taining to the belly, hence opposite to the dorsal side). The plane of 

 symmetry passes through the two ends, through the dorsal and ventral 

 surfaces, and between the right and left halves. 



Some animals possess a number of planes of symmetry. If these 

 planes all have a certain straight line in common, that line is the axis 



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