754 



TEXTBOOK OF ZOOLOGY 



which g-ives the species its name. If pork containing trichina cysts 

 is eaten by a man, the cysts are digested off in the stomach, the larvae 

 become active and penetrate the mucosa of the small intestine to 

 moult. They soon become full grown, sexually mature adults, some- 

 times beginning copulation only forty hours after being swallowed. 

 The adult worms are short-lived and are not harmful to the host, but 

 each fertile female, about a week or ten days after the infected meat 

 was swallowed, begins to produce thousands of microscopic larvae 

 which she deposits in the walls of the intestine, usually directly into 



Fig. 396. — Larvae of Trichinella spiralis, encysted in voluntary muscle. The 

 adults are parasites of the intestine. (Photomicrograph by Albert E. Galigher, 

 Inc.) 



a lymph vessel or blood vessel. The larvae are carried by the blood 

 or lymph to the heart, and from there they are carried by the blood 

 to all parts of the body. The larvae which reach voluntary muscle 

 enter the muscle fibers and coil up into spirals, grow rapidly to a 

 len^h of about 1 mm., and in a few weeks become enclosed in a cyst 

 of connective tissue which grows around them. Trichina larvae are 

 likely to be most abundant in active muscles, such as the diaphragm, 

 the intercostals, and the muscles of the larynx, tongue, and eye. The 



