46 



ELEMENTS OF BIOLOGY. 



and which are known as " serial homology " and *' lateral 

 homology." 



Serial homology is established by the presence in a single 

 animal of a succession of two or more parts which are 

 placed in a longitudinal series one behind the other, and 

 which have the same fundamental structure. In no animals 

 is this phenomenon better seen than in the A?tfmlosa, such 

 as the great majority of the Crustaceans, in which it is easy 

 to see that the body is composed of a longitudinal succes- 

 sion of rings or segments, placed in a row one behind the 

 other, and essentially alike in their structure (fig. 14). In 



Fig. 14. — Fairy Shrimp {Chirocephahis diaphamts). After Baird. 



the majority of cases, however, whilst these serial parts have 

 a fundamentally identical structure, and are clearly built 

 upon a common plan, they are not all alike ; but they are 

 modified in different regions of the body to fit them for the 

 fulfilment of special functions. Certain of the segments, 

 therefore, differ physiologically from certain others, and thus 

 come to differ morphologically as well. There are other 

 cases, however, as the Centipedes (fig. 15), for instance, in 



Fig. 15. — Centipede (.S"ri7//7/i?«</r<7). After Jones. 



which the greater number of the serial parts are exactly 

 similar both in structure and in function; and these, per- 



