MORPHOLOGY OF ANIMALS 



Finally this specialization of the somites is carried a step 

 farther in higher arthropods and vertebrates, in which we 

 have an intimate fusion of metameres and a coalescence of 

 organs in certain regions, such as to more or less com- 

 pletely mask the fundamental segmentation. This is espe- 

 cially true of the vertebrates, the lower forms of which 

 show segmentation of the axial skeleton (vertebrae and 

 ribs) and attached muscles, of the nerves, of the gills and 

 their blood vessels, and of the excretory and sexual organs ; 

 while in the higher vertebrates (reptiles, birds, and mam- 

 mals) segmentation is limited in the adult to the axial 

 skeleton, muscles, and nerves. The fusion of somites is 

 most pronounced in the anterior part of the body; such 

 fusion leads to the formation of a head {cephalization). 

 The head of insects contains three or four somites (Fig. 

 15, A), while the vertebrate head is composed of not fewer 

 than nine. 



Primitively the limbs are all alike and a pair is borne on 

 each somite (many annelids) ; however in higher annelids 

 and arthropods they disappear entirely from certain 

 somites and in others undergo great modifications of struc- 

 ture to fit them for particular functions. In the case of 

 vertebrates they are limited to but two pairs, and it is 

 probable that these are derived from a continuous lateral 

 limb-ridge by the suppression of an intermediate portion. 



The great modifications and complications which have 

 been briefly sketched lead far from the simple form of 

 the gastrula, which is the ground-form of all Metazoa. 



1:22 n 



