THE ANIMAL BODY 129 



significance, because it indicates a basic structural identity in 

 the body plan of all these forms. It is exhibited in the 

 developmental process in each generation, even though the 

 adult body in the various groups differs widely in form and 

 arrangement of organs. Such a state of affairs clearly sug- 

 gests a genetic relationship throughout the whole animal 

 series the origin of the diverse forms by evolution. 



D. CRAYFISH 



Bearing in mind the general plan of organization and 

 development of the body of the Earthworm, we must next 

 consider briefly the main principle underlying the changes 

 in this plan which give rise to many of the diverse forms 

 among the higher Invertebrates. This principle appears to 

 be chiefly a specialization of the individual segments so that 

 the body, instead of consisting of a large number of essen- 

 tially similar metameres, is formed of a linear series of meta- 

 meres, many of which are quite different from the rest. 

 Moreover, by the partial or complete fusion of two or more 

 metameres and the suppression of segmentation, definite 

 regions of the body are delineated. This principle is well 

 illustrated by animals of the group known as the ARTHRO- 

 PODA, or pointed-footed' Invertebrates, such as Lobsters, In- 

 sects, Millipedes, and Spiders. Altogether the Arthropoda 

 comprises nearly half a million living species. 



The body of a primitive Arthropod differs from that of the 

 Earthworm chiefly in the reduction of the number of seg- 

 ments and the development of paired jointed appendages as 

 outgrowths from the body in each segment. (Fig. 70.) From 

 such a type all the multitude of diverse forms of Arthropod 

 bodies can be derived. For instance, in the CRAYFISH, which is 

 essentially a fresh-water Lobster, the body consists of nine- 

 teen segments, of which segments 1 to 5 together form the 



