THE CLASSIFICATION OF ANIMALS 37 



The word " Worm " is a merely popular term which is used 

 to denote any wriggling creature of simple structure, and 

 amongst the so - called 

 " Worms " are included 

 members of many different 

 phyla. The Flat- worms 

 agree with Ccelenterates 

 in possessing a mouth which 

 is used both for ingesting 

 food and for ejecting waste 

 material, and in having as 

 the basis of their structure 

 a ramified set of tubes lined 

 by an internal layer of cells 

 and contained within an 

 outer layer or skin. Bu-t 

 between skin and internal 

 layer is contained a mass of 

 cells which is differentiated 

 into definite muscular and 

 nervous tissues to a degree 

 which never occurs in any 

 Ccelenterate ; and in par- 

 ticular there are aggrega- 

 tions of nervous matter con- 

 stituting two large ganglia 

 termed the brain, and 

 two cords or bands along 

 the sides of the body. We 

 are thus presented with a FIG. 10. A, diagrammatic longitu- 



definite concentration of 



nervous matter which is 



termed a central nervous 



system; and such a system, 



consisting of the brain and 



the two cords, is found in 



no Coelenterate and is 



characteristic of all Platy- 



helminthes. The phylum Platyhelminthes includes many 



kinds of animals which live in the interior of other animals, 



dinal section of a simple Coelen- 

 terate with a bud attached ; B, 

 a stinging-cell (cnidoblast) with 

 a nematocyst inside ; (7, a dis- 

 charged nematocyst ; b, bud ; ect, 

 ectoderm ; end, endoderm ; j, 

 jelly ; n, nucleus ; nem, nemato- 

 cyst ; o, mouth ; s.h., sense-hair; 

 t, tentacle. 



