LARVAL TAPEWORMS IN MAN 247 



age shrinks and a depression appears at the opposite end; a 

 large portion of the body is occupied by a pyriform central mass. 

 The full development is reached in two or three weeks, the larvae 

 being then about 0.5 mm. long. Small trout fed on the infected 

 copepods were successfully infected. The passage through the 

 intestine and body cavity of the fish is slow; it requires about six 

 days for the larvae to reach the liver in young fish, and in older 

 fish it may take from two to four weeks. In the fish the worms 

 develop into plerocercoid larvae in the muscles. Pike, perch and 

 other carnivorous fish are most frequently infected. 



When eaten by a susceptible host in raw or imperfectly cooked 

 fish, the larva, except the head, is digested, and the head, at- 

 taching itself to the wall of the small intestine, begins to grow 

 into an adult worm at the rate of about 31 to 32 proglottids a 

 day. The mature eggs begin to appear in the faeces within a 

 month. 



The fish tapeworm is especially active in the production of 

 toxins which cause intense anemia. Its head has been found 

 to produce oleic acid, a substance which has blood-destroying 

 properties. Often the nervous symptoms produced by this 

 species are also very marked. 



Two other species of Dibothriocephalidae have been found in 

 man. One of these, Diphyllobothrium cordatus (Figs. 84G and 

 87D), occurs in dogs, seals and other fish-eating animals in Green- 

 land. It only accidentally establishes itself in man. Diplo- 

 gonoporus grandis is a very large species, found in Japan, in 

 which there is a double set of reproductive organs. The genital 

 openings are arranged in two longitudinal grooves on the ventral 

 side of the worm (Fig. 84F). This species is rare in man. 



Larval Tapeworms in Man 



There are several species of tapeworms which inhabit the human 

 body in the larval or bladderworm stage. Three types are found 

 in man. Most important are the huge multiple cysts or " hyda- 

 tids " of Echinococcus granulosus, a small tapeworm of dogs. 

 Second, there are the bladderworms of the common pork tape- 

 worm, Tcenia solium, which often occur in large numbers, and 

 may be of very serious nature if located in important organs. 

 And, finally, there are two species of Sparganum. This is not 



