236 THE STUDY OF ANIMAL LIFE CHAP. 



spends part of its life inside a water-snail. The cercarise 

 pass from the snail (Planorbis, Bulinus) into the water, and 

 they are able to enter man through minute cuts in the 

 skin of hands and feet. 



Class : Cestoda or Tapeworms. These are all internal 

 parasites, and, with the exception of one (Archigetes), 

 which fulfils its life in the little river-worm Tubifex, the 

 adults always occur in the food-canal of backboned 

 animals. Like the flukes, they have adhesive suckers, 



ft/ 



and sometimes hooks as well ; unlike flukes and planarians, 

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

 hosts through their skin, and have no mouth or gut. 

 Like the endo-parasitic flukes, the tapeworms have 

 (except Archigetes) intricate life-histories. Both Tur- 

 bellarians and Trematodes are small, rarely more than an 

 inch or so in length, but the tapeworms may measure 

 several feet. In the adult Tcenia solium, which is some- 

 times found in the intestine of man, we see a small head 

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

 wall of the faod-canal ; it buds off a long chain of " joints,' 1 

 each of which is complete in itself. As these joints are 

 pushed by continued budding farther and farther from 

 the head, they become larger, and distended with eggs, 

 and even with embryos, for the hermaphrodite tapeworm 

 is able to fertilise its own ova a very rare thinof 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, where they eventually burst. The 

 microscopic 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 larvae (hexacanths) 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 

 the nineteenth century that they were recognised as the 

 young stages of the tapeworm. For if the diseased pig be 

 killed and its flesh be eaten half-cooked by man, then 

 each bladder- worm -may become an adult sexual tape- 



