CHAPTER X 



STRUCTURE AND CLASSIFICATION OF FLAT-WORMS 

 (Platyhelmia) and THREAD-WORMS (Nemathelmia) 



FLAT-WORMS (Platyhelmia) 



The animals included in this phylum are unsegmented worm- 

 like creatures which in most cases are markedly flattened from 

 above downwards, hence the name of the group. They are 

 divided into three classes: — i. Tape- Worms (Cestoda); 2. Flukes 

 (Trematoda) ; 3. Planarian Worms (Turbellaria). 



Class I.— TAPE -WORMS (Cestoda) 



Tape- Worms are all internal parasites, and in the adult stage 

 are almost without exception inhabitants of the food -tube in 

 various Vertebrate animals, from Man downwards. The structure 

 has undergone profound modification as a result of the parasitic 

 habit, and the life-history is often complicated for the same reason. 

 Details must be deferred till the phenomenon of parasitism is fully 

 discussed, it being sufficient for the present purpose to briefly 

 consider an average example of the group, the Common Tape- 

 WorTfi [Tcsnia solium) (fig. 272), the adult stage of which is found 

 in the human intestine. The long flattened whitish body has 

 earned the name of " tape "-worm for the animal, which may be 

 as much as 3 yards in length. It consists of a very small head 

 and neck followed by the trunk, which gets broader and broader 

 as we pass back to the hinder end, and is divided into a very large 

 number oi joints, which must not be mistaken for true segments 

 such as have been described for annelids. It is indeed doubtful 

 whether a tape-worm should be regarded as a single individual. 

 An alternative view is that the head and neck together represent 

 what may be called the primary individual, from which a long 

 series of secondary individuals, the joints, has been originated. 



Close examination of the minute head by means of a lens 



