140 



ELEMENTS OF BIOLOGY 



TENTACLE 



HEAD 



A PPENDA GE ^^. 





Fig. 84. — Nereis, the com- 

 mon sandworm of the sea 

 shore. In the living animal 

 four eyespots may be dis- 

 tinguished in the dorsal sur- 

 face of the head. The broad 

 appendages are composed of 

 two lobes. The upper lobes, 

 visible in the figure, contain 

 many blood vessels and 

 serve as respiratory organs. 

 The lower lobes are tra- 

 versed by stiff setae and serve 



motion. No such ideal arrangement is 

 found, for in all known metameric ani- 

 mals various segments are modified in 

 various ways to serve special functions. For 

 example, in the earthworm several of the 

 most anterior segments that appear during 

 the development of the animal fuse to- 

 gether to form the leading and correlating 

 element, the head. This fusion of anterior 

 segments into a single unit, the head, is 

 termed cephalization. The number of seg- 

 ments involved in forming the head varies 

 in different varieties of annelids and 

 among other segmental forms. The num- 

 ber of segments that have gone to form 

 the human head is a matter of dispute, for. 

 the process of cephalization in this highly 

 modified form is obscure, but indications 

 are that the process has taken place. 



The Principle of Homology. In 

 some annelids, for example the sandworm, 

 Nereis (Fig. 84), each segment, except the 

 most anterior, is equipped with a pair of 

 lateral appendages. Each appendage is 

 comparable with the others, being derived 

 in the same way from the same materials 

 during development. Such structures that 

 have the same origin are termed homolo- 

 gous structures. Thus the ca?lom is homol- 

 ogous throughout the series of forms in 

 which it is developed; the wing of a 

 bird and the fore limb of a cat, being 



as organs of locomotion. 



