

CHAPTER II. 



ON THE TAPE-WORM. 



THE tape-worms (Cestoidea) constitute a peculiar group of 

 entozoa which only attain their perfect development and sexual 

 maturity in the intestinal canal of vertebrate animals. Those 

 that are often met with in other internal organs than the 

 intestinal canal, in fishes, reptiles, birds, or mammals, or in the 

 interior of inferior animals, are always sexually undeveloped. 

 In this sexless state the tape-worms wait for an opportunity to 

 pass out, which occurs when the creature they lodge in is swal- 

 lowed by some vertebrate carnivore. It is only when such sexless 

 tape-worms have thus passively effected their entrance into the in- 

 testinal canal of the appropriate Vertebrata, that their sexual matu- 

 rity takes place, and they become capable of laying eggs for further 

 propagation. In this wandering the remarkable circumstance 

 occurs, that whilst these undeveloped tape- worms pass into the 

 stomach of the predacious animal in a more or less uninjured 

 condition, and establish themselves in its intestine, the soft parts 

 of their former host yield to the digestive juices. Numerous 

 examples attest the truth of this assertion, but of these I will 

 only select the following. 



In certain neighbourhoods the sticklebacks are infested by a 

 kind of teenioid parasite which lies free in the cavity of the 

 abdomen, and often distends the body to an unusual size. This 

 parasite has been before described under the name of Bothrio- 

 cephalus solidus. In the stickleback its joints and sexual appa- 

 ratus are undeveloped and always remain so. 



In the intestine of many of the water fowl which prey upon 

 these sticklebacks, a sexually matured tape-worm, known to 

 naturalists by the name of Bothriocephalus nodosus, has been 

 found. This is no other than the Bothriocephalus solidus in a 

 further stage of development ; after its former host, the stickle- 

 back, has been digested in the bird's stomach, it is released, 



