246 



THE TAPEWORMS 





plerocercoid type, i.e., they have long wormlike bodies with an 

 invaginated head at one end (Fig. 86C). 



Fish Tapeworm. The common fish tapeworm of man, 

 Dibothriocephalus latus, is an important species in the districts 

 in which it occurs. It is found in all countries where fresh-water 

 fish is extensively eaten, and especially in countries where it is 

 commonly eaten raw. In the Baltic countries, Switzerland, 

 Russia, Japan, and about the Central African lakes this parasite 

 is particularly common. Relatively few cases have been re- 

 ported in the United States, though the larvae are said to be found 

 frequently in fish from the Great Lakes. 



The fish tapeworm is a large species and commonly reaches a 

 length of from six to 30 feet, or even more, 

 with from 2000 to 4200 short, broad pro- 

 glottids, only the terminal ones of which 

 are as long as broad. The scolex (Fig. 87C) 

 is almond-shaped. Unlike the tapeworms 

 of the family Taeniidae, the genital openings 

 are near the middle of the under surface of 

 the proglottids, instead of at one side. In 

 the ripe proglottids (Fig. 84E) the uterus 

 is in the form of a rosette near the center 

 of the segment. The proglottids do not 

 usually retain the eggs until they break off 

 from the chain, but void them, as do flukes, 

 through the genital pore. The empty pro- 

 glottids, shrunken and twisted, are broken 

 i, ciliated em- off in short chains from time to time. 



The eggs (Fig. 93 A), which are large and 

 brown v/ith a lid at one end as in fluke 

 contain six-hooked embryos which are furnished with a 

 covering of cilia (Fig. 93B). The eggs hatch in water after several 

 weeks and the embryos swim for a time by means of their cilia, 

 though they often slip out of their ciliated envelope and creep 

 on the bottom. It is believed that the embryos first enter some 

 small aquatic animal, probably a crustacean, which is eaten by 

 a pike or perch or other carnivorous fish. The larvae, which are 

 of the plerocercoid type, develop in the muscles of these fish. 



When eaten by a susceptible host in raw or imperfectly cooked 

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



FIG. 93. 



B 



An egg of fish 



( f Af S t a eLoi) ab 



