PARASITES AND PARASITIC DISEASES OF DOGS 29 



segments are often seen and referred to as rectal worms or pin worms. 

 The common double-pored tapeworm has a larval stage in fleas and 

 the biting lice of dogs. The fleas become infected while they are flea 

 larvae feeding on the organic matter in trash, the larval fleas swallow- 

 ing the tapeworm eggs in this trash. Biting lice swallow the eggs 

 while feeding on the contaminated skin of the dog. In these insects 

 the tapeworm eggs hatch and the escaping embryo develops to a 

 larval tapeworm in the body of the insect. When dogs, annoyed by 

 the itching and irritation due to these insects, hunt them out and 

 swallow them, the tapeworm larvae become adult tapeworms in the 

 intestine of the dog. This tapeworm sometimes occurs in man, 

 especially in children, as a result of the accidental swallowing of 

 infested fleas or lice by persons. 



A larger group of tapeworms is made up of forms which have only 

 two circlets of hooks (fig. 27) and have a genital pore on only one side 

 of each segment (fig. 28). The thick-shelled eggs do not occur in egg 

 capsules but in a uterus from which some eggs escape into the intesti- 

 nal contents and feces. The eggs (fig. 29) occur as isolated specimens 



Figure 28.— The gid tapeworm, MuUiceps multiceps. Entire worm. Actual size. From 



Hall, 1910. 



in the feces. The segments, usually containing only a part of the 

 original egg content, escape in the feces and in time break up, releasing 

 the remaining eggs, which are washed about on the ground and on 

 vegetation by the rain. When such eggs are swallowed by suitable 

 hosts, such as sheep, cattle, swine, hares, rabbits, etc., the hosts varying 

 with different species of tapeworms, the eggs hatch and each releases 

 an embryo which penetrates the walls of the digestive tract and 

 develops in some tissue (liver, lungs, muscles, brain, intermuscular or 

 subcutaneous connective tissue, etc.) to form a larval tapeworm or 

 bladderworm (fig. 30). When these bladderworms are eaten by 

 dogs, the tapeworm head in the bladderworm resists digestion and 

 becomes the head of a tapeworm in the intestine of the dog. 



Other dog tapeworms include such forms as the broad fish tape- 

 worm, Diphyllobothrium latum, which has 2 slitlike suckers in place 

 of the 4 rounded suckers of the preceding forms, and which has its 

 intermediate stages in fish. Dogs become infected by eating infested 

 fish. The broad fish tapeworm also occurs in man. 



Cats may harbor the double-pored dog tapeworms, Dipylidium 

 caninum and D. sexcoronatum, the broad fish tapeworm, Diphyllo- 



