INTRODUCTION TO THE METAZOA 



89 



In asymmetrical animals, the body can- 

 not be divided by planes into similar parts; 

 in other words, the body has no definite 

 form or arrangement of parts. Many proto- 

 zoans and most sponges are asymmetri- 

 cal. 



A radially symmetrical animal possesses a 

 number of similar parts called antimeres, 

 which radiate out from a central axis like 

 the spokes of a wheel. It is possible to draw 

 a number of planes through a central axis 

 dividing the body of these animals into equal 

 parts (Fig. 44). The hydra (Fig. 44) is an 

 example; its tentacles are similar and radiate 

 out from the mouth. Some simple sponges 

 (Fng. 46), the majority of the coelenterates 

 (Fig. 61), and most of the adult echino- 

 derms are radially symmetrical. Radial sym- 

 metry is best suited to sessile animals, since 

 the similarity of the antimeres enables them 

 to obtain food or repel enemies from all 

 sides. 



The body of a bilaterally symmetrical 

 animal is so constructed that the chief or- 

 gans are generally arranged in pairs on either 

 side of an axis, passing from the head (an- 

 terior end) to the tail (posterior end). 

 There is only one plane through which the 

 body can be divided into two similar parts. 

 An upper or dorsal surface and a lower or 

 ventral surface are recognizable, as well as 

 right and left sides. In most of the bilateral 

 animals, the anterior end is differentiated 

 into a head, which contains a concentration 

 of nervous tissue and which is supplied with 

 numerous sense organs. This modification 

 is termed cephalization. Bilateral symmetry 

 is characteristic of the most successful ani- 

 mals living at the present time, including all 

 vertebrates and most invertebrates. 



Some animals are spherical, as, for exam- 

 ple, certain Protozoa. Such an animal shows 

 approximate spherical symmetry. It is 

 symmetrical around the axis of a sphere 

 like a ball. It can be divided into two similar 

 parts by a cut in any direction through the 

 center. 



Spherical synmietr}' is disadvantageous. 



since such an animal can show only an in- 

 definite kind of locomotion. Most spherical 

 animals are free-floating as the radiolarians; 

 or they progress by a rolling movement as 

 the volvox. 



It is doubtful if perfect s\mmctry is to be 

 found ann\hcrc in the animal kingdom. 

 Animals said to show spherical symmetry 

 usually only approach a spherical form. 

 There are traces of bilateral symmetry in 

 the various radially symmetrical animals. Al- 

 though the human form is considered a 

 good example of bilateral symmetry, every- 

 one knows that the right and left sides of the 

 human body are not identical. Nevertheless, 

 the zoological concept of symmetn,' is of 

 great importance in the study of animals; 

 this will become evident as the different 

 groups are studied. 



Metamerism 



Metameric animals have bodies composed 

 of more or less similar parts, or they have 

 organs arranged in a linear series along the 

 main axis. Each part is called a metamere, 

 somite, or segment. In many animals meta- 

 merism is not shown by the external struc- 

 tures, but is exhibited by the internal or- 

 gans; this is true of the vertebrates, which 

 have the vertebrae of the backbone, the 

 ribs, and nerves metamerically arranged. The 

 earthworm (Fig. 91) is a good illustration 

 of both external and internal metamerism; 

 the body consists of a great number of simi- 

 lar segments; and the ganglia of the nerve 

 cord, the chambers of the bodv cavitv, the 

 blood vessels, and the excretory organs are 

 segmcntally arranged. 



The earthworm may serve also as an ex- 

 ample of an animal with homonomous seg- 

 mentation, since the mctameres are similar. 

 The crayfish (Fig. Ill), on the other hand, 

 is a heteronomous animal, since division of 

 labor has resulted in dissimilarity of the 

 mctameres of different regions of the body. 

 The vertebrates, including man, are all 

 heteronomous. 



