130 ORGANS AND ORGAN SYSTEMS 



body and various organs are the result of its mode of life, or 

 whether it adopted this mode of life because of its peculiar 

 structures. The same problem recurs in connection with all 

 types of living things and may be expressed by the question: 

 does the environment and mode of life of an animal type cause 

 the race to become adapted to its surrounding conditions or does 

 the animal type choose the environment most suitable to its 

 peculiar structures? We may leave the discussion of this 

 question for the present with the non-committal statement that 

 all animals are more or less perfectly adapted to the conditions 

 of their environment, and will take up the problem of the 

 origin of such adaptations in a later chapter. 



B. REGIONAL DIFFERENTIATION. A superficial examination 

 of the worm is sufficient to show that it has quite definite 

 structures. 



Metamerism. The entire body is divided by faint ring-like 

 constrictions or annuli into short segments called somites or 

 metameres, which on first view seem to be all alike. These 

 rings are characteristic of a great group of worms called An- 

 nulata or annelids from this peculiarity and all are metameric 

 animals in which only slight modifications of the metameres 

 occur. With metamerism, however, the possibilities of differen- 

 tiation are almost unlimited and we find that all of the higher 

 types of animals with the exception of the phyla of the soft- 

 bodied molluscs and spiny-skinned Echinoderms are built on 

 this plan of structure. It is plainly evident in Crustacea, in- 

 sects, fish, and snake, but is limited to the vertebral column in 

 the majority of vertebrates. 



Antero-posterior Differentiation. Even in the earthworm, 

 where the metameres seem to be all alike, there is some regional 

 differentiation. If the mouth end of the worm be tickled it 

 will be found to be more sensitive than the middle region or 

 the opposite end. If a bright light is suddenly thrown on the 

 mouth end the worm will react vigorously. This end of the 

 worm therefore is more irritable than the other. Furthermore 

 at or near this end there are several external openings of 

 internal organs not found elsewhere; thus on the eighth, ninth, 



