CHAPTER XI 



THE LIVER-FLUKES, TAPE- WORMS, AND OTHER 

 PARASITIC FLAT WORMS 



The Worms. In the older classifications the name Vermes 

 was applied to a large group of worm-like creatures which 

 resembled each other in some respects, but as these animals 

 came to be better known it was found that some were not at 

 all closely related to others and it became necessary to divide 

 the group into five smaller groups of equal rank, the flat worms 

 (Platyhelminthes,) the round worms (Nemathelminthes), the 

 rotifers (Trochelminthes), the sea-mats and lamp-shells 

 (Molluscoidea) and the segmented worms (Annelida). These 

 all differ from the polyps and sponges by being bilaterally 

 symmetrical and in having three well-developed layers of 

 cells in the body-wall, the mesoderm being well developed. 

 Usually, also, the various systems of organs are much more 

 complex, and the individuals do not form colonies. 



The first group (the branch Platyhelminthes; Gr. platus, 

 broad, helmins, worm) includes the liver-flukes, tape-worms 

 and other parasitic flat-worms, and a few free living flat worms 

 that live in fresh water or salt water, or, more rarely, on land. 

 They are usually much flattened and without true segmenta- 

 tion, although the bodies of the tape-worms have a jointed 

 appearance owing to their being largely made up of a string of 

 reproductive units. Many of the parasites have very com- 

 plex life histories and live in different kinds of animals while 

 passing through their successive stages of development. 



Fresh-water Planarians. In the mud at the bottom of 

 ponds of fresh water or clinging to the rocks or sticks in 

 such places are often found small flat creatures that are known 

 as planarians. They look somewhat like small leeches, which 

 belong to the branch Annelida, but may be distinguished 



69 



