No. 646] TAPEWORM INFECTION 



443 



cysted (Figs. 4 and 5) along the walls of the digestive 

 tract, particularly on the stomach, or they may be found 

 burrowing freely amongst the visceral structures, or 

 within the surrounding muscular walls. In appearance 

 they are translucent, whitish or yellowish-white organ- 

 isms which may vary from a few millimeters to about 

 twenty millimeters in length. They are long, slender and 

 worm-like in character (Figs. 5 and 6). At the anterior 

 end is the head (Fig. 7), which possesses two lateral slits. 

 This head end is constantly changing its shape in the liv^- 

 ing specimens, becoming slender and spear-like at one time 

 and stouter and knob-like at another time. The body 

 proper of the larva may undergo periodic contractions 

 and extensions. Covering its entire outer surface are 

 stiff, bristle-like structures which, at first glance, seem to 

 resemble cilia, but which do not possess any independent 

 motion. Posteriorly the body tapers off into a blunt 

 rounded margin (Fig. 6). 



The damage done to the fish by these larval tapeworms 

 is considerable. In the first place, the fish lose their 

 healthy appearance, becoming much thinner and paler in 

 hue. The parasitic larvse undoubtedly produce injurious 

 toxins which interfere with the proper functions of the 

 host. Then, again, the burrowing habits of these para- 

 sites injure the tissues of the fish, causing them to become 

 mushy. And finally, secondary infections of a serious 

 sort may develop within the injured portions. As a re- 

 sult of all this damage great numbers of the fish die. 



The life history of these larval tapeworms is extremely 

 interesting. Those who are familiar with tapeworm in- 

 fection know that ordinarily two organisms are necessary 

 for the completion of the life history. The adult tape- 

 worm lives in one animal called the primary host, where- 

 as the larval tapeworm dwells in another animal called 

 the secondary host. The primary host becomes parasi- 

 tized by eating the infected portions of the secondary host. 



In the case of the tapeworm under consideration it is 

 quite obvious that the trout acts as the secondary host. 



