200 INTRODUCTION TO THE WORMS 



be recognized externally by the segmentation of the body; 

 the internal anatomy is totally different. Both sexes are repre- 

 sented in each individual. 



The number of different species of worms in these three phyla 

 which have been found in man is well up in the hundreds. In 

 the following pages each group of these worms which contains 

 important human parasites will be dealt with, but only those 

 species which are important, or which are particularly inter- 

 esting from some other point of view, will be individually 

 considered. 



Parasitic Habitats. — As to the parts of the body which may 

 be attacked by worms of one kind or another, there is hardly 

 any organ or tissue which is exempt. There are flukes which 

 habitually infest the intestine, liver, lungs and bloodvessels, 

 and one species occasionally wanders to the muscles, spleen, 

 brain and many other organs. The adult tapeworms are all 

 resident in the small intestine, but larval tapeworms are found 

 in various locations in the body. The majority of the parasitic 

 nematodes of man are found in the intestinal canal but there 

 are exceptions to this. The adult Trichinellce, for instance, 

 inhabit the intestine, but the larvae are found in the muscles; 

 the adult Filarioe usually live in the lymph vessels, whereas the 

 larvae swarm in the blood; the guinea- worm and some other 

 nematodes creep under the skin in the connective tissue; the 

 lungworm of the hog, Metastrongylus apri, which occasionally 

 occurs in man, infests the lungs and bronchial tubes; and Dioc- 

 tophyme renale {or Eustrongylus gigas) is an occasional human 

 parasite which occurs in the kidneys and rarely in the body cavity. 

 The leeches, on the other hand, are parasitic on the smface of 

 the body or in the cavities of the nose and mouth. 



Life History and Modes of Infection. — The life history and 

 mode of infection of worms varies with the habitat in the body. 

 Every parasitic worm must have some method of gaining access 

 to the body of its host, and must have some means for the escape 

 of its offspring, either eggs or larvae, from the host's body in 

 order to continue the existence of its race. Many species utilize 

 intermediate hosts as a means of transfer from one host to an- 

 other; others have a direct life history, i.e., they either develop 

 inside the escaped egg and depend on such agencies as food and 

 water to be transferred to a new host, e.g., pinworm, or they 



