FARMEKS' INSTITUTES. 



399 



Fig. 3. 



Distonium lanceolatum. 



the smallest of all arise in the tissues of the animal. These vessels (See Fig. 2) 

 axQ supposed to be excretory in their function. Some of these flukes are her- 

 maphrodites. These animals at two stages of their development exist in water, 

 hence are most dangerous when the affected animals are kept in damp pastures. 

 The liver flukes of sheep (Fig. 3) and 

 •cattle are the most noted example. These 

 parasites cause the disease known as the 

 "rot," "water rot," and "fluke rot." 

 Omitting the Acantliocepliala (thorn- 

 headed worms) as of less interest to 

 our present purpose, I proceed to the 

 last order. 



Nematodes (Bound Worms). 



These more highly organized parasitic worms are familiar to us in the pin- 

 worms of children, and the notorious trichina spiralis. 



In this order there is a comjjlete alimentary canal, the sexes are distinct, 

 ■while in form they are generally round and slim, the head being at the smaller 

 end. As in the tape-worms, there are two forms, representing different ages, 

 which are very different in appearance, habits, function, and surroundings. 

 These abound in nearly all parts of mammals, birds and fishes. One thousand 

 species have been described, about one dozen of which may infest man. Next 

 to those already mentioned, the common round worm of man is most commonly 

 noticed in this department of literature, and is probably the most apt to attract 

 attention of all. 



The most dangerous species of this order, in fact of all human parasites, is 

 the Iricliina sjnralis. Let us then leave the general discussion of worms, and 

 confine our attention to the pork-worm. We will first consider its 



Natural History. 



The mature animal (Fig, 4) is a small round worm, found only in the intes- 

 tines of the following animals : The hog, man, dogs, cats, 

 rats, mice, etc. The male is 1-18 of an inch long, while 

 the female is much longer, attaining -J of an inch. The 

 head end of both sexes is tapering, while the posterior 

 extremity is abrupt. At the apex of the tapering extrem- 

 ity of this thread-like worm is the minute mouth. Follow^- 

 ing this is the oesophagus, flask-shaped stomach, and intes- 

 tine which, ends with the rectum. In the male, as in birds, 

 the rectum unites with the seminal ducts into a common 



duct or cloaca. The interior male 



or^an 



IS seen, even m 



"Fir' A o -" 



Adult rric7i»n«. -Highly ^1^® Jo^^ig. Iji^g ^1^ the posterior part of the body, and con- 

 magnifled. " sists of a closed sac, which with maturity ends in the sem- 

 inal duct. The rectum, which receives this, bends forward after leaving the 

 body, thus forming the external male organ, which lies between two lobular 

 appendages. The female possesses an ovary which is visible even in the young. 

 When mature there is also a uterus, in which the eggs hatch, and an external 

 opening which is located one-quarter the length of the animal from the head. 

 The ovary occupies the posterior one-third of the body, and in it of course, and 

 along its entire length are developed the eggs. The uterus extends from the 

 ovary to the external opening. These animals are ovoviviporous then, as the 

 animals come forth after hatching takes place. 



