34 



TEXT-BOOK IN ENTOMOLOGY 



forward direction would gradually evolve a fore and aft, dorsal 

 and ventral, and bilateral symmetry. As soon as this was attained, 

 as the effect of creeping over rough irregular surfaces there would 

 result mechanical lateral strains intermittently acting during the 

 serpentine movements of the worm. The integument would, we can 

 readily suppose, tend to bend or yield, or become permanently 

 wrinkled, at more or less regular intervals. The arrangement of 

 the muscles would gradually conform to this habit of creeping, and 



finally the nervous system and 

 other organs more directly con- 

 nected with the creeping move- 

 ments of the organism would tend 

 to be correlated in their arrange- 

 ment Avith that of the segments. 

 In this way the homonomous seg- 

 ments of the annelid body prob- 

 ably became developed, and their 

 relations and shapes were eventu- 

 ally fixed by inheritance. After 

 this stage was reached, and limbs 

 began to appear, the segments 

 would tend to become heterono- 

 mous, and to be grouped into 

 regions. 



The origin of the joints or seg- 

 ments in the limbs of arthropods 

 was probably due to the mechanical 

 strains to which what were at first 

 soft fleshy outgrowths along the 

 sides of the body became sub- 

 jected. Indeed, certain annelid 

 worms of the family Syllidee have 

 segmented tentacles and para- 

 podia, as in Dujardinia (Fig. 19). 

 We do not know enough about the habits of these worms to under- 

 stand how this metamerism may have arisen, but it is possibly due 

 to the act of pushing or repeated efforts to support the body while 

 creeping over the bottom among broken shells, over coarse gravel, 

 or among seaweeds. 



It is obvious, however, that the jointed structure of the limbs of 

 arthropods, if we are to attempt any explanation at all of the origin 

 of such structure, was primarily due mainly to lateral strains and 



FIG. 19. Dujrtrdiiiid rot-ifera, with 

 jointed tentacles and caudal appendages. 

 With some changes, after Qiuitivtatrrx 



