40 PRIMITIVE ANIMALS 



the Arthropoda the body is protected from external 

 agencies by the hard external skeleton to which the 

 muscles are attached ; the internal organs are there- 

 fore protected from external agencies and from the 

 strain of muscular contraction, and do not require 

 the protective body-cavity. 



The presence of a coelom system distinct from the 

 vascular system may also be of importance in resisting 

 the infection of micro-organisms from the alimentary 

 canal, since the coelomic fluid is 'full of amoeboid 

 corpuscles whose function is to attack and devour 

 any bacteria or poisonous organisms which may find 

 their way into the coelom. 



If it is true that the Arthropods are derived from 

 Annelid-like ancestors, we are in need of some definite 

 evidence to support the theory that the coelom in the 

 Arthropods has been displaced by the swelling of the 

 veins, and that the remains of it are to be sought in 

 the reproductive and certain excretory organs. 



It is here that a particular living animal gives us 

 the clue we want, a living animal that has survived 

 from a distant age and has retained certain primitive 

 features of organization which help us to piece to- 

 gether the course of events leading to the disappear- 

 ance of the coelom in the higher Arthropods. The 

 PERIPATUS (Fig. 6), for so the animal is called, is 

 represented by a small number of closely related 

 species which live in various parts of the tropics and 



