As many as fifty of the cysticercoids have been found in the body cavity of a single flea. When the dog takes up an infested flea or louse, by biting itself, or when the cat licks them up, the larvæ quickly develop into tapeworms, reaching sexual maturity in about twenty days in the intestine of their host. Puppies and kittens are quickly infested when suckling a flea-infested mother, the developing worms having been found in the intestines of puppies not more than five or six days old. [Illustration: 116. Dipylidium caninum. Cysticercoid. After Villet.] Infestation of human beings occurs only through accidental ingestion of an infested flea. It is natural that such cases should occur largely in children, where they may come about in some such way as illustrated in the accompanying figures 117 and 118. _Hymenolepis diminuta_, very commonly living in the intestine of mice and rats, is also known to occur in man. Its cysticercoid develops in the body cavity of a surprising range of meal-infesting insects. Grassi and Rovelli (abstract in Ransom, 1904) found it in the larvæ and adult of a moth, _Asopia farinalis_, in the earwig, _Anisolabis annulipes_, the Tenebrionid beetles _Akis spinosa_ and _Scaurus striatus_. Grassi considers that the lepidopter is the normal intermediate host. The insect takes up the eggs scattered by rats and mice. It has been experimentally demonstrated that man may develop the tapeworm by swallowing infested insects. Natural infection probably occurs by ingesting such insects with cereals, or imperfectly cooked foods. [Illustration: 117. One way in which Dipylidium infection in children may occur. After Blanchard.] _Hymenolepis lanceolata_, a parasite of geese and ducks, has been reported once for man. The supposed cysticercoid occurs in various small crustaceans of the family Cyclopidæ. [Illustration: 118. The probable method by which Dipylidium infection usually occurs.] Several other cestode parasites of domestic animals are believed to develop their intermediate stage in certain arthropods. Among these may be mentioned: _Choanotænia infundibulformis_, of chickens, developing in the house-fly (Grassi and Rovelli); _Davainea cesticillus_, of chickens, in some lepidopter or coleopter (Grassi and Rovelli); _Hymenolepis anatina_, _H. gracilis_, _H. sinuosa_, _H. coronula_ and _Fimbriaria fasciolaris_, all occurring in ducks, have been reported as developing in small aquatic crustaceans. In these cases, cysticercoids have been found which, on account of superficial characters, have been regarded as belonging to the several species, but direct experimental evidence is scant. ARTHROPODS AS INTERMEDIATE HOSTS OF NEMATODE WORMS