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ESSENTIALS OF ZOOLOGY 



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 w^eek 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 

 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 w^hich reach voluntary muscle 



Fig. 189. — Larvae of Trichinella spiralis, encysted in voluntary muscle. The 

 adults are parasites in the intestine, (Photomicrograph by Albert K. Galigher, 

 Inc.) 



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

 length 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 

 symptoms of trichinosis (the disease caused by trichina worms) vary 

 according to the stage of the infection. In the first week, when the 

 adult females are burrowing into the walls of the intestine to de- 

 posit their larvae, the symptoms are so much like those of typhoid 



