THE SYMMETRY OF ANIMAL FORMS. 83 



A large number of animals have a peculiar kind of 

 symmetry, consisting in the repetition along the longi- 

 tudinal axis of parts or joints that are almost exactly 

 similar. These similar parts are called Segments or 

 Metameres ; and an animal possessing this kind of 

 structure is called a Segmented animal. Familiar 

 examples of segmented animals are the earth-worm, 

 with successive rings ; the centipede, the caterpillar, or 

 the lobster, with successive rings which bear legs. 

 An animal with a row of pairs of legs, like the lobster 



Fi^. 22,— A segmented animal, belonginpf to the Cra«tacea, tbe Opossum 

 Shrimp, jtfi/sis vulgaris. The body has Buccesisive metameres, or joints, and 

 pairs of appendages. 



or centipede, usually has some of them modified in 

 some way to serve varitjus purposes other than that 

 of walking : thus, some of the lobster's legs carry gills 

 to breathe with ; some walk, and some nip. Legs of 

 this kind are referred to as appendages, a term 

 that covers all movable structures attached to a body- 

 ring, whatever their special use. 



Many animals that have not an obvious metameric 

 structure bear traces of it in some of their organs. 

 Thus a fish exhibits a segmented type by its rows 

 of successive masses of muscles ; and indeed all the 

 vertebrates show something of the kind ia the succes- 



