272 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



of appendages of a still lower form is given in Fig. 174. 

 In this case the appendages are too much alike to be 

 easily classified. Often, too— e. g., in the Limulus— the 

 appendages which in the higher crustaceans are walking 

 appendages become swimmers, while the maxillipeds be- 

 come walkers. Finally, in the lowest crustaceans and, 

 still better, in the centipede and in some marine worms, 

 the rings and appendages are greatly increased in num- 

 ber, and are so similar that there is not modification 



Fig. 173. — Vibilia, an amphibod crustacean. (After Milne Edwards.) 



enough to furnish argument for homology. There is 

 only a slight modification of the head and tail joints 

 (see Fig. 175). Indeed, some marine worms multiply 

 by self-division. In such cases at the point of division 

 some of the rings are consolidated and appendages 

 modified to form a new head and. a new tail (Fig. 176). 



Going up the Scale. — Going up the scale to in- 

 sects the modification is greater, but the elements — viz., 

 rings and appendages — are the same. An insect consists 

 ideally of about sixteen or seventeen segments and ap- 



