108 PRINCIPLES OF ANIMAL BIOLOGY 



Metamerism. — Animals exhibiting metamerism are composed of a 

 linear series of body segments fundamentally alike in structure. These 

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

 are said to be segmented or metameric. In simple metameric animals the 

 somites closely resemble one another in size, form and in 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. 83 and 142) is a metameric animal. 



It is composed of a series of ring-like 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 He 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 nervous system. The sexual organs also have 

 a segmental arrangement although they are Umited to a few somites. 

 Certain other organs are repeated in only a few segments, but in general 

 the earthworm's structure is that of a metameric animal. 



In complex animals the metameric arrangement has often become 

 obscured through fusion of somites, loss of organs and centralization. 

 However, the primitive arrangement is readily seen in the embryos of 

 such animals. Thus the embryos of the vertebrates generally reveal a 

 well marked metamerism in certain organs (the muscles, for example), 

 in which this arrangement is later partly or completely lost. Not all 

 metamerism has been lost even in the adults of vertebrates, however, for 

 in the mammals, for example, metamerism may be seen in the vertebra, 

 ribs, spinal nerves and ganglia and branches of the dorsal aorta (artery). 



Metamerism is interpreted by some biologists as indicating an-carher 

 colony formation, the somites representing the individuals of the colony. 

 It is not improbable, however, that rhythmical processes occurring 

 during development are responsible for the division of the body into 

 similar parts, and that the ancestors of metameric animals were never 

 colonial. 



Internal Organization. — The organization of cells into tissues, tissues 

 into organs and organs into systems has been described in the preceding 

 chapter and repetition of this account is avoided as far as possible here. 

 It is important, however, in relation to problems discussed later, to de- 

 scribe in more detail tissues, gland-formation, and the general morphology 

 of the systems of organs as they exist in the more complex animals. 



Tissues may be classified with reference to their general function into 

 four types, epithelial, sustentative, contractile, and nervous. All animal 

 tissues may be referred to one or another of these general types which are 

 discussed briefly below with reference to their structure and functions. 



Occurrence and Function of Epithelium. — This kind of tis>^ue covers 

 all outer and inner surfaces of the body. It is composed of simple cells 



