360 MANUAL OF NATUEAL HISTORY. 



pernicious parasite, well known to farmers as 

 the cause of the fatal disease termed "stag- 

 gers " in sheep, is the Ccenurus cerebralis, and 

 is found in the brain. One very curious form, 

 Trichina spiralis, has been discovered embedded 

 in the muscles of the human frame ; the Tcenia 

 soliuwi is the species of Tape-worm most frequently 

 met with in the intestinal passages of man ; and 

 another pest belonging to the same class is the 

 Fluke (Fasciola hepatica) which infests the livers of 

 sheep and other ruminating animals, and causes the 

 destructive malady named the " rot/' The Vertum- 

 nus thethydicola is an example of the propriety of 

 changing the name of Entozoa into " Parenchyma- 

 tous-Parasites," for, although in structure it agrees 

 with the other animals of the class, it does not live 

 in the interior, but is parasitic on the skin of a Nu- 

 dibranch Mollusk, the Thethys fimbriata. 



VII. CLASS. PARENCHYMATOUS- 

 PARASITES (Sterelmintha.) 



Alimentary canal ramified in substance of animal, 

 with only one aperture, oral and anal ; mouth with- 

 out radiating tentacles ; sexes mostly united. Para- 

 sitic in other animals. 



I. ORDER HOOK-WORMS (Acanthocephala.) 



Body cylindrical or sacciform ; mouth in form of 

 a retractile proboscis, armed with recurved hooks ; 

 sexes distinct. 

 1. FAMILY. Hook - Worms (Echinorhynchidae.) 



