244 AN INTRODUCTION TO ZOOLOGY. 



existence of successive pairs of gills (or in the higher 

 vertebrates of structures which are their homologues), 

 and of two successive pairs of limbs ; while the suc- 

 cessive pairs of nerves given off from brain and spinal 

 cord, the successive joints of the backbone itself, and 

 the successive pairs of ribs associated with some of 

 them, are among other traces of metameric structure, 

 like the muscle-masses already named. These, al- 

 though, in all Vertebrates higher than fishes, their 

 original character is more or less lost, are seen in the 

 embryo of higher forms : for instance, in an early 

 stage of the developing chick, successive pairs of 

 masses of tissue are seen, from which the muscles of 

 the body are afterwards developed. These muscle- 

 plates are sometimes called somites, because they 

 are believed to be comparable with the successive 

 body-rings of a segmented animal. Some think, how- 

 •ever, that the metamerism of the vertebrate body, in- 

 stead of being an indication of ancestral structure, 

 may have arisen independently from the working of 

 rthe tendency to the repetition of parts consequent on 

 the elongated form of the body. 



The Cranial Nerves. The skull is believed to 

 consist of a number of successive parts which are pro- 

 bably to be regarded as metameric, nobody knows how 

 many, all fused together. Some trace of these fused 

 segments is seen in the successive pairs of nerves given 

 off by the brain ; but some of these, too, are probably 

 formed by the union of several pairs into one. The 

 brain gives off twelve pairs of nerves, of which the first 



