CHAPTER VI 



MORPHOLOGY OF HIGHER TYPES 



The preceding chapter was devoted to the aggregation of cells into 

 colonies, the organization of cells of the metazoan body into various 

 tissues, the general aspects of organs and systems, and the aggregation 

 of metazoan individuals into organic colonies. Only a generalized ac- 

 count of grades of organization, leading to the conception of a scale of 

 complexity which is perhaps of evolutionary significance, was there 

 attempted. When details of structure especially in the higher animals 

 are examined there is found, of course, great diversity. However, this 

 diversity of form and structure may be reduced to a relatively small 

 number of general plans which are discussed in this chapter. The 

 existence of a community of plan is of importance from the standpoints 

 of function, development, classification, and evolution. 



Symmetry. — Despite the great diversity of form existing among ani- 

 mals it is a striking fact that the members of most species exhibit some 

 degree of symmetry. The term symmetry is used in its geometrical 

 sense, according to which a figure which possesses symmetry may be 

 divided by a line or plane in such a way that one half is a mirrored image 

 of the other. If in such a figure any point be chosen on one side of the 

 line or plane of division a corresponding equidistant point may be found 

 on the opposite side. Animals which can be thus divided by a plane 

 into two mirrored halves are said to possess symmetry while all others 

 are asymmetrical (not symmetrical). The symmetry of animals is of 

 three well marked types, bilateral, radial, and universal. 



Bilateral Symmetry. — Most animals of common acquaintance as the 

 dog, horse, fish, crayfish, clam and earthworm possess a symmetry such 

 that there can be only one plane which divides the animal into mirrored 

 halves. Anterior (front) and posterior (rear) ends are different, and the 

 dorsal (upper) and ventral (lower) surfaces are different, so that only 

 the two sides are alike. In such animals the plane of symmetry must be 

 passed through the antero-posterior axis of the body and through points 

 on the mid-dorsal and mid-ventral lines. In this manner left is divided 

 from right. Animals of this type with only two like sides exhibit bilat- 

 eral symmetry, that is, two-sided symmetry. This is a very common 

 type of symmetry, especially in the higher animals. 



Radial Symmetry. — Among certain groups of lower animals in which 

 differentiation has not progressed as far as in the ones cited above, the 

 form of the animal is almost a true cylinder or a modified cone. Ex- 



105 



