340 BIOLOGY: GENERAL AND MEDICAL 



Tsenia echinococcus. All of these have com- 

 plex life histories requiring an alternation of 

 hosts. 



Class Turbellaria. Of these Rhabdocoela is 

 parasitic in the kidney of certain gasteropods; 

 Fecampia, in the gut of certain crabs 

 Phylum Nemathelminthes. 



Class Nemertinea. Of these worms few are 

 parasitic. Malacobdella, however, is parasitic 

 in Lamellibranchs. 



Class Nematoda. These are the round worms, 

 of which some species live independent lives, 

 though perhaps the greater number are parasitic. 

 A few infest plants, most of them animals. 

 Their distribution is so broad that few of the 

 higher animals escape them, and they extend 

 from the arthropoda through the whole series 

 to man himself. Of the human parasites of 

 this class the Ascaris lumbricoides or round 

 worm, theOxyurisvermicularis orpin-worm, the 

 Anchylostoma duodenale or the palisade worm, 

 the Necator americana or hook-worm, and the 

 Trichocephalus dispar or whip-worm are intes- 

 tinal parasites. Trichinella spiralis is at one 

 stage an'intestinal parasite, but later a muscle 

 parasite. Filaria bancrofti is a blood parasite 

 and Filaria medinensis, a tissue parasite. All 

 are of rather frequent occurrence. It is not in 

 all cases possible to separate commensalism 

 and mutualism from parasitism in discussing 

 these worms. 



Class Acanthocephala. This contains four 

 genera and a number of species, all of which are 

 parasitic. They infest arthropods for the most 

 part during the immature stage, the adults 

 appearing in vertebrates, especially fishes 

 where they frequent the intestine. They are 

 rarely found in man. 



