BASIC ORGANIZATION OF THE METAZOA 



79 



In general, animals which move rapidly or are capable of well coordi- 

 nated movements are bilateral. The radial animals are usually slow 

 movers and frequently are attached to fixed objects. Universally sym- 

 metrical animals are aquatic and progress with a rolling movement. 



Asymmetry. — Any object which cannot be divided into corresponding 

 halves by any plane is said to be asymmetrical. Many of the protozoa 

 are made asymmetrical by a groove running spirally part way round the 

 body. The coiled shell of a snail is asymmetrical. 



Fig. 65. — ^Various coelenterates, showing their radial symn^etry. A, sea anemone; 

 B, group of coral polyps; C, the medusa, Mitrocoma cirrata, ventral view. D, polyp of the 

 hydroid, Perigonimus serpens. {A and B after Jordan, Kellogg, and Heath; C after Mayer; 

 D after Allman.) 



Many animals which are externally symmetrical may have their 

 internal structures arranged on an asymmetrical plan or on a plan of 

 symmetry different from the external plan. Examples are the heart, 

 stomach, and other parts of the alimentary tract and the lobes of the 

 liver in man, which are arranged asymmetrically. Many animals which 

 exhibit asymmetry in certain of their adult organs are symmetrical in 

 early stages of development. The flatfishes (halibut, floimder, and sole) 

 which have two eyes placed on one side of the head, are in their early 

 embryos bilaterally symmetrical, but one eye migrates through the head 

 to its new^ position. 



Metamerism. — Animals exhibiting metamerism are composed of a 

 linear series of body segments fundamentally alike in structure. These 

 units are called somites or metameres, and animals so constructed are 

 said to be metameric. In simple metameric animals the somites closely 

 resemble one another in size, form, and the arrangement of organs. In 

 no animal, however, are all somites entirely alike because some of them 

 have become specialized and perform special duties. 



The common earthworm (Figs. 135, 137) is a metameric animal. 

 It is composed of a series of ringlike somites outwardly much alike. 

 The limits of the somites are marked on the outside by grooves, and on the 

 interior by the septa (cross partitions) which lie immediately under the 

 grooves. The segmental arrangement extends to both external and 

 internal structures and involves organs of locomotion and excretion, 

 muscles, blood vessels, and the nervous system. The sexual organs also 

 have a segmental arrangement, although they are limited to a few somites. 



