PARASITIC FLATWORMS 369 



marine species and all from hosts commonly frequenting the sea 

 and most likely to become infected there. 



The parasitic flatworms fall readily into two great classes, the 

 Trematoda or flukes and the Cestoda or tapeworms. Some authors 

 would make a third intermediate group out of the few forms which 

 are known as Cestodaria and resemble the flukes in having a simple 

 body and the tapeworms in details of internal anatomy. In this 

 work they are treated with the tapeworms. As apart from these 

 few cases flukes and tapeworms can be fairly readily distinguished, 

 it is advantageous for the student to have each group treated sep- 

 arately in a distinct section of the chapter; and to this treatment 

 the following brief synopsis may serve as an introduction. 



Body soft, flattened, shaped more or less like a simple scale, leaf, band, 

 or ribbon. ... ... Phylum Plathehninthes. 



The external surface may have hooks, spines, or scales, or be 

 provided with warts or rugosities, but it does not possess a tough, 

 shiny, smooth, resistant cuticula. In a few cases the body is cylin- 

 drical, conical, or spindle-shaped and does not display the charac- 

 teristic flattening mentioned in the key. 



Intestine present . ... Class Trematoda . page 369. 



Intestine absent . Class Cestoda page 424. 



Sometimes the intestine is so rudimentary or so thoroughly con- 

 cealed by other organs that its presence is difficult to determine. 

 It is, however, the only absolute diagnostic characteristic which in 

 the last analysis separates a fluke from a tapeworm. 



Trematoda 



The trematode or fluke is usually flattened, oval, seed-shaped, 

 or rarely rodlike, attenuate, or globular in shape. With few ex- 

 ceptions one finds on the surface one or more cup-shaped suckers. 

 The number and arrangement of these constitute a means of sub- 

 dividing the group. Careful examination under magnification dis- 

 closes pores or openings and also in some cases hooks or spines on 

 the surface. Many of the flukes are transparent and permit the 

 observer to identify the main internal organs. 



The alimentary system which usually starts at the forward tip 



