288 THE RELATIONS OF ANIMALS TO DISEASE 



These develop within the intestine of the pig, but soon make their 

 way into the muscles or other tissues, where they are known as 

 bladder worms. If man eats undercooked pork containing them, 

 he is likely to become a second host for tapeworms. Another 

 common tapeworm parasitic on man {Tmnia saginata) lives part 

 of its life as an embryo within the muscles of cattle. The adult 

 tapeworm consists of a round headhke part provided with hooks, 

 by means of which it fastens itseK to the wall of the intestine 

 (figure on page 240) . This head now buds off a series of segment- 

 Hke structures, which are practically bags full of sperms and eggs. 

 These structures, caUed proglot'tids, break off from time to time, 

 thus allowing the developing eggs to escape. The proglottids 

 have no separate digestive systems, but the whole body surface, 

 bathed in digested food, absorbs it and is thus enabled to grow 

 rapidly. 



Roundworms. — Still other wormlike creatures called round- 

 worms are of importance to man. Some, as the vinegar eel found 

 in vinegar, or the pinworms parasitic in the lower intestine, partic- 

 ularly of children, do httle or no harm. The as'caris, a larger 

 roundworm, sometimes infests children but is rarely dangerous to 

 its host. 



The pork worm or trichina (tri-ki'na), however, is a parasite 

 which may cause serious injury. It passes through the first 

 part of its existence as a parasite in a pig or other vertebrate 

 (cat, rat, or rabbit) ; it is covered within a tiny sac or cyst, in the 

 muscles of its hosts (figure on page 240). If undercooked pork 

 containing these cysts is eaten by man, the covering is dissolved 

 off by the action of the digestive fluids, and the living trichina 

 becomes free in the intestine of man. Here it reproduces, deposit- 

 ing the young on the other side of the intestinal wall; and the 

 young migrate into muscles, causing inflammation there and 

 resulting in a painful and often fatal disease known as trichino'sis. 

 The government inspects all animals killed for food in large 

 slaughterhouses, so that there is little likelihood nowadays of 

 becoming infected with trichina. Pork from private slaughter- 

 houses may be infected, however, so it is safer to cook pork 

 thoroughly, so that any parasites will be killed before the meat 

 is eaten. 



