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, 

 Diphyllobothrium 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, 

 FIG. 93. An egg of fish through the genital pore. The empty pro- 

 ll^S^S^ad glottids, shrunken and twisted, are broken 

 opercuium;j3, ciliated em- off in short chains from time to time. 



bryo of same. X about rm /-n- rvo A \ i i i i 



300. (After Looss.) The e gg s ( Fl g- 93A )> wmch are large and 



brown with a lid at one end as in fluke 



eggs, contain six-hooked embryos which are furnished with a 

 covering of cilia (Fig. 93B). The eggs hatch in water in from eight 

 to fifteen days, and the embryos swim about by means of their 

 cilia, though they often slip out of their ciliated envelope and 

 creep on the bottom. The embryos have recently been shown 

 by Rosen and by Janicki to develop in certain species of Cyclops 

 and Diaptomus, copepod crustaceans, into larvae which are 

 called procercoids. In 12 to 15 days these larvae are about 0.4 

 mm. (^ of an inch) in length, sausage shaped, with a spherical 

 caudal appendage carrying the embryonic hooks. This append- 



