ONTOGENESIS 



135 



of segments in the body apparently has no influence 

 on or relation to the individuality of the organism. 

 In the higher animals this primitive segmentation 

 has become overshadowed by the unity of the whole, 

 so that no segment could be sacrificed without 

 destroying the individual. This type of bodily 

 structure is called metameric segmentation, and is the 

 fundamental plan in nearly all the animal phyla, 1 



FIG. 49. Diagram of metameric segmentation (as of the earth- 

 worm) : A, longitudinal section of the body showing the alimentary 

 canal and coelom divided by septa ; B, cross-section ; coe, coelom ; al, 

 alimentary canal ; m, mouth ; an, anus. (From Sedgwick and Wilson.) 



from the generalized Annelids, in which the segments 

 bear to the body somewhat the same relation that 

 the individual cars do to a vestibuled train, to 

 the higher vertebrates, in which the segmental 

 feature is plainly evident only in the developmental 

 stages. 



Fission in Lower Plants. In the important 

 group of the Bacteria, multiplication takes place 

 exclusively by fission. In the spherical type (Coc- 

 cus) the planes of division may be repeatedly 



1 The exceptions are the higher Mollusca, Flatworms, Coelenterates, 

 and Sponges. 



