216 ZOOLOGY. 



worms — surrounded by an umbrella or cup-shaped 

 expansion. Here belongs : 



The Trichina {Trichina spiralis). 



In the sexual condition this lives as a minute worm 

 (male one-seventeenth, female one-eighth to one-sixth 

 of an inch long), the so-called " intestinal trichina," in 

 the gut of human beings and carnivorous mammals. 

 In this situation it brings forth living young (some 

 fifteen hundred in number), and the multiplication is 

 enhanced by the fact that of the very numerous 

 trichinae found in the gut there are, on the average, 

 about twelve females to every male. The young 

 trichinse bore into the walls of the gut, and are carried 

 by the blood-stream into the muscles. At first each 

 little worm extends itself longitudinally in a muscle 

 fibre, but, as it gets larger, curves and becomes spirally 

 twisted, so that the sheath (sarcolemma) of the fibre 

 is bulged more and more outwards. The growth of 

 the young trichina is very rapid — an individual only 

 ^^ of an inch long before leaving the gut may attain 

 the length of one twenty -fifth of an inch in fourteen 

 days. It then grows no more, and the sheath of the 

 muscle-fibre gradually thickens to form a capsule or 

 cyst. Later on, in about a year, lime is deposited in 

 the capsule. The "muscle triehiuEe" retain their 

 vitality for several years. If now the host of the 

 muscle trichina is devoured by any other mammal, 

 the capsule is digested in the stomach of the latter, 

 and the once more liberated worm becomes a sexually 

 mature " intestinal trichina " in a few days. The pig 

 is the ordinary host of trichinae, which (without the 

 intervention of any other host) can be permanently 

 parasitic in this domestic animal, since pigs often 

 devour swine's flesh. They often enough devour even 

 their own young, and, especially in large slaughter- 

 houses, swine are often fed with the offal of their 



