24 ON PARASITES. 



mous caudal vesicle, which in animals has been seen as large as a 

 child's head. The hooks are arranged in two rows upon the tetra- 

 gonal head ; the neck is somewhat slender, whence the name. A 

 tsenia in all respects like the taenia marginata has been produced in 

 the dog by feeding that animal with the cystic worm. 



The second order is not found in the human subject. The best 

 known representative is the Echinorhynchus gigas, which occurs in 

 the small intestine of the hog and various other animals. The me- 

 tamorphoses of the order are not yet known. 



I omit a description of this order and proceed with that of the 

 third. 



Oedbe III. Teematoda. 



These are solitary animals, mostly hermaphrodite. They have 

 median or lateral suctorial pores. The alimentary canal is usually 

 branched (rarely single). Evolution is mostly accomplished by me- 

 tamorphoses, and very often by alternate generation. 



This is a very extensive and very interesting order, but does not find 

 its habitat to any great extent in man. Two families only have been 

 found in man. 



Family 1. Mokostoma. 



The body is soft, elongated, polymorphous, flattened, or slightly 

 rounded. The head is continuous or discrete with a neck. The 

 mouth is terminal or anterior, acetabular, crenulate, armed or unarmed. 

 The genital aperture is distinct and double, the male anterior to the 

 female acetubular. The penis is protractile. The female aperture 

 is small and inconspicuous. Habitat : Mammals, birds, amphibia 

 and fishes. Always outside the alimentary tract, and either free 

 or enclosed in sacs. Metamorphoses and alternate generation occur 

 as in the next. 



Family 2. Distoma. 



The body is flattened or somewhat rounded. Anteriorly there is 

 a circular sucker or disc in which the mouth opens ; posteriorly there 

 is another sucker. The two suckers serve to enable the animal to 

 attach itself firmly to the vascular structures, from which it derives 

 nutriment. The posterior disc is sessile or pedunculated, and placed 

 at various distances from the caudal extremity. The generative 

 organs occupy a large portion of the body. 



