CHAPTER VI 

 PHYLUM PLATHELMINTHES 



The Plat hel mint hes comprise a phylum of wormlike animals 

 with bodies usually flattened, devoid of body cavity, and lacking 

 true segmentation. The bulk of the body is composed of a mass 

 of connective tissue termed parenchyma within which the various 

 organs are embedded. Externally, this is covered by either a 

 ciliated epithelium or a non-cellular cuticula. A body muscula- 

 ture is present, the fibers of which are usually circular, longitu- 

 dinal, and diagonal in arrangement. Frequently, in addition, 

 fibers are also found penetrating the body from dorsal to ventral 

 surfaces. The nervous system usually consists of two or more 

 longitudinal strands which, near one end of the body, bear a pair 

 of ganglia called the brain. In many instances, the longitudinal 

 nerve trunks are connected by cross-commissures which give 

 the system a ladderlike appearance. 



Typically, the digestive system is composed of an ectodermal 

 pharynx and a mesenteron, for the proctodaeum is lacking. 

 Exceptions to this are found in the cestodes, which lack all evi- 

 dence of a digestive system, and in the nemertines, which have a 

 complete system terminating posteriorly in an anus. A few 

 species of trematodes possessing an anus have also been described. 

 Excretion is by means of a system of tubules, the protonephridia, 

 which ramify throughout the body and end in minute structures 

 called the flame cells. 



Most of the flatworms are hermaphroditic. The reproductive 

 organs consist not only of the primary organs or gonads but to 

 them are added many accessory organs and glands not found in 

 lower organisms. There is great diversity in methods of repro- 

 duction in this phylum. The usual bisexual method is fre- 

 quently supplemented by fission, budding, and parthenogenetic 

 development. 



Form, habits, and details of structure differ so profoundly in 

 the members of the various classes that few features beyond those 

 already mentioned are held in common by all members of this 

 phylum. 



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