40 



ZOOLOGY 



Between the muscular layer and the digestive tube or gut 

 there exists a space, or rather a set of inter-communicating 

 spaces, which we may term the body-cavity. They are filled 

 with a clear fluid which surges about as the animal wriggles ; 

 in this fluid are contained the products of digestion which 

 have been exuded from the digestive cells, and the whole thing 

 is the prototype of the " blood " of the higher animals. In 

 order to distinguish it from body-cavities of a different kind, 

 this type of body-cavity, the simplest and the first we have 

 encountered, is termed a primary body-cavity, or a blood 

 body- cavity or haemocoele. Nematoda vary in size from 



FIG. 13. A Nema- 

 tode worm; o, 

 position of 

 mouth. 



7HUSC 



FIG. 14. Diagrammatic cross- 

 section of a Nematode worm ; 

 cut, cuticle ; g, gut ; A, hsemo- 

 ccele or primary body-cavity ; 

 muse, muscle cells. 



minute microscopic "worms" to creatures a yard long. 

 These last, the dreaded Guinea Worms of the tropics, infest 

 the skin and underlying tissues of men and animals, and 

 cause dangerous sores and ulcers. It has been suggested 

 that they may have been the " fiery flying serpents " which 

 attacked the wandering Israelites. The only safe way to 

 extract them is to tie their protruding ends to a stick, and then 

 slowly and gently to wind them round this. If sudden extrac- 

 tion be attempted, the worm breaks, and the portion left in the 

 sufferer dies and decays, and causes poisoning and death. It 

 is therefore possible that the famous "serpent on the pole" 

 exhibited by Moses was an object-lesson to his followers as to 



