68 TKICHINOSIS, 



the male is very degenerate and lives in the uterus of the 

 female, as many as three or four males being found in one 

 female. 



This "degeneration" is a very common phenomenon in 

 parasitic life. Any part of an animal that is not used we find 

 degenerates, and may become lost entirely unless nature utilises 

 it for some other purpose. The second pair of wings in the 

 true flies (Diptera) have apparently degenerated, doubtless 

 through disuse ; but they are 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 ex- 

 tinction 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. spiralis). — 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 

 aU 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. 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 Trichina 

 spiralis produces living young. 



