RELATION OF INSECTS TO THE PARASITIC WORMS 53 



and Trematoda, or flukes; the latter into Nematoda, or roundworms in 

 the restricted sense, Gordiacea, or horse-hair worms, and Acanthocephala, 

 or thorn-headed worms. 



CESTODA OR TAPEWORMS 

 f 



All tapeworms whose life history has been well established require an 

 intermediate host, and are thus heteroxenous parasites. A typical life 

 history of a tapeworm is as follows : The adult lives in the intestine of 

 the final host. The eggs pass out of the body of the infested animal in 

 the feces. The feces or food or drink contaminated by them are swallowed 

 by an animal that can act as an intermediate host. The eggs thus 

 reaching the intermediate host hatch in the alimentary tract and the 

 embryos set free migrate into nearby or remote tissues of the body, 

 developing finally into an intermediate stage, commonly of the type known 

 as a cysticercoid, in the case of those tapeworms whose intermediate stages 

 occur in insects. Having reached this stage further development of 

 the parasite awaits the time when the intermediate host or infested por- 

 tions of its body are swallowed by an animal that can act as the final host, 

 whereupon it resumes its development and, becoming mature, completes 

 the life cycle. About 100 species of tapeworms are known whose adult 

 stages occur in man or domestic animals. Four of these, Dipylidivmi 

 caninum (the double-pored tapeworm of the dog, cat, and man), Hymen- 

 olepis diminuta (the yellow-spotted tapeworm of rats, mice, and man), 

 Hymenolepis nana (the dwarf tapeworm of rats, mice, and man), and 

 Choanotcenia infundibulum (one of the tapeworms of the domestic fowl), 

 have insects as intermediate hosts, with the possible exception of the dwarf 

 tapeworm, in whose life history the part played by insects has not been 

 definitely determined. 



Dipylidium caninvm (Linnaeus, 1758) Railliet, 1892 



This tapeworm, sometimes called the double-pored dog tapeworm, is of 

 very common occurrence in the small intestine of dogs and cats, and of 

 occasional occurrence in human beings. Its larval stage (cysticercoid) 

 occurs in the biting dog louse [^Trichodectes latus (canis)'\ as deter- 

 mined experimentally by Melnikov (1869), and in fleas (CtenocepJialus 

 canis, C. felis, and Pulex irritans). Fleas apparently are the usual 

 intermediate hosts. Grassi and Rovelli (1888, 1889) followed the various 

 stages of larval development in adult fleas, from the hexacanth embryo 

 to the fully developed cysticercoid, and as they failed to find the parasite 

 in larval fleas concluded that only adult fleas can act as hosts. Recently, 

 however, Joyeux (1916) has reached the conclusion that adult fleas 



