76 TKICHINOSIS. 



still represented by the so-called " balancers," — small knob-like 

 processes that are not used now as organs of flight, but as 

 specially modified organs of sense, organs of equilibrium. Very 

 many parasites having no use for limbs, we find are devoid of 

 them (parasitic Isopod Crustacea). One pair of the gill-arches 

 seen in fish, in the higher animals have nearly gone ; but they 

 have been saved from total extinction by being converted into 

 structures for a different object — namely, for forming the bony 

 chain in the auditory capsule. Use and disuse increase and 

 decrease respectively the size and development of any part of 

 the animal frame. Perhaps the greatest degeneration is in some 

 of the worms, where the male, as mentioned above, is reduced 

 simply to a testis, which lives permanently in the female's body. 



The Trichina of Pork (T. sinralin). — This small worm pro- 

 duces the often fatal malady Trichinosis. The adult sexual 

 worm is found in the intestine of man, the pig, the rat, and in 

 practically all other carnivorous animals. The adult worm is 

 only about one-twelfth of an inch in length. The sexes, as in 

 all Nematodes, are quite distinct, and a true copulation takes 

 place. The result of this union is that a very large number 

 of fertile ova are laid by the female, each female laying, it is 

 said, at least a thousand eggs at one time. The females may 

 live for five or six weeks. We find on examining these worms 

 that to every male there are at least twelve females : we can 

 thus realise the hosts of ova that are laid. Not unusually the 

 female Trichinella spiralis produces living young. 



The ova soon hatch in the alimentary canal of the host, and 

 the larvae derived from them commence to lay eggs about six or 

 seven days after their entry. Often by the second day after 

 ingestion the larvae commence to mature ; by the fourth day 

 they become mature. The young worms when they come from 

 the ova are extremely minute cylindrical bodies, only -jj-^th 

 of an inch in length. They soon commence to bore their way 

 through the walls of the intestine and enter the sarcolemma 

 of the muscles, penetrating into the primitive bundles, the sub- 



