CHAPTER XII 



FORMS OF ANIMALS 



Animals of various kinds seem to present a great variety of forms, 

 but when these are carefully studied it becomes possible to recognize 

 a small number of distinct types. 



78. Asymmetry. — Symmetry is regularity of form and involves the 

 existence of corresponding parts. In the case of some animals, particu- 

 larly the more simple ones, there seems to be no symmetry; this condition 

 is termed asymmetry, and the animals are spoken of as being asymmetrical 

 (Fig. 22). 



79. Spherical, or Universal, Symmetry. — An ideal form of symmetry 

 which is rarely approached in nature would be a form in which an indefi- 

 nite number of planes might be passed through the center of the animal 

 and each plane exhibit a structure precisely similar to that of every other, 

 as well as dividing the animal into symmetrical halves. This is termed 

 spherical, or universal, symmetry. This is most nearly attained in some of 

 the one-celled animals (Fig. 31 D). 



80. Radial Symmetry. — Another form of symmetry which is presented 

 by many of the lower many-celled animals is one in which the body can 

 be divided by a number of radial planes into parts that are similar to 

 each other. This type of symmetry is termed radial symmetry and the 

 parts, since they are opposite around the center, are termed antimeres 

 (Fig. 108). 



81. Bilateral Symmetry. — Other animals are capable of being divided 

 by a single median plane into similar right and left halves, the one being 

 a mirror image of the other. This is termed bilateral symmetry and is 

 characteristic of higher forms. 



82. Metamerism. — Many bilaterally symmetrical animals have a 

 body which is not divided into similar parts other than the right and left 

 halves. Others, however, including the highest animals, are divided into a 

 series of parts arranged in a linear series which, because they are placed 

 one behind another, are termed metameres, or segments, the condition 

 being termed metamerism. If these metameres are, generally speaking, 

 similar to each other, it is known as homonomous metamerism, which is 

 well illustrated by the common earthworm or angleworm (Fig. 144). 

 If, however, these parts are dissimilar, the condition is termed heterono- 

 mous metamerism; an example of this type is the crayfish (Fig. 162), in 



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