230 The Study of Animal Life PART m 



adhesive suckers, and sometimes hooks as well ; unlike flukes and 

 planarians, which have a food-canal, they absorb the juices of their 

 hosts through their skins, and have no mouth or gut. Like 

 the endo-parasitic flukes, the tapeworms have (except Archigetes) 

 intricate life -histories. Both Turbellarians and Trematodes are 

 small, rarely more than an inch at most in length, but the tape- 

 worms may measure several feet. In the adult T&nia solium, 

 which is sometimes found in the intestines of man, we see a small 

 head like that of a pin ; it is fixed by hooks and suckers to the 

 wall of the food-canal ; it buds off a long chain of "joints," each 

 of which is complete in itself. As these joints are pushed by con- 

 tinued budding farther and farther from the head, they become 

 larger, and distended with eggs, and even with embryos, for the 

 bisexual tapeworm seems able to fertilise itself, which is a very rare 

 thing among animals. The terminal joints of the chain are set free, 

 one or a few at a time, and they pass down the food-canal to the 

 exterior. The tiny embryos which they contain when fully ripe 

 are encased in firm shells. It may be that some of them are eaten 

 by a pig, the shells are dissolved away in the food-canal, small six- 

 hooked embryos emerge. These bore their way into the muscles of 

 the pig and lie dormant, increasing in size however, becoming 

 little bladders, and forming a tiny head. They are called bladder- 

 worms, and it was not till about the middle of this century that they 

 were recognised as the young stages of the tapeworm. For if the 

 diseased pig be killed and its flesh eaten (especially if half-cooked) 

 by man, then each bladder-worm may become an adult sexual tape- 

 worm. The bladder part is of no importance, but the head fixes 

 itself and buds off a chain. For many others the story is similar ; 

 the bladder- worm of the ox becomes another tapeworm (Tcenia 

 saginata) in man ; the bladder-worm of the pike or turbot becomes 

 another (Bothriocephalus latus] ; the bladder-worm of the rabbit 

 becomes one of the tapeworms of the dog, that of the mouse passes 

 to the cat, and so on. A bladder-worm which forms many heads 

 destroys the brain of sheep, etc. , and has its tapeworm stage ( Tcznia 

 ccenurtis) in dog or wolf. Another huge bladder- worm, which has also 

 many heads, and sometimes kills men, has also its tapeworm stage 

 (Tania echinococcus) in the dog. But enough of these vicious 

 cycles. 



2nd Set of Worms. Ribbon Worms or Nemerteans 



4th Class, Nemertea. In pleasing contrast to the flukes and tape- 

 worms, the Nemerteans are free -living " worms." They are 

 mostly marine, often brightly coloured, almost always elongated, 

 always covered with cilia. There is a distinct food-canal with 

 a posterior opening, a blood-vascular system for the first time, 

 a well-developed nervous system, a remarkable protrusible "pro- 



