CESTODA OR TAPEWORMS. 157 



budded off a long chain of joints. The last of the joints or 

 proglottides, is liberated (singly or along with others) and 

 passes down the intestine of its host to the exterior. It 

 has some power of muscular contraction, and is distended 

 with little embryos within firm egg-shells. When the pro- 

 glottis ruptures, these cases are set free. 



In certain circumstances, the embryos, within their firmly 

 resistent egg-shells, may be swallowed by the omnivorous 

 pig. Within the alimentary canal of this animal the egg- 

 shells are dissolved, and the embryos bearing six anterior 

 hooks are liberated. They bore their way from the intestine 

 into the muscles or other structures, and there encyst. 

 They increase in size and become passive, vegetative, 

 asexual " bladderworms." A bud from the wall of the 

 bladder or proscolex grows into the cavity of the same, 

 and forms the future " head " or scolex. It is afterwards 

 everted, and then the bladderworm consists of a small head 

 attached by a short neck to a relatively large bladder. But 

 this remains quiescent, and without power of further develop- 

 ment, unless the pig be eaten by some other Vertebrate. 



When man unwittingly eats " measly " pork, that is pork 

 infested with bladderworms, an opportunity for further 

 development is afforded. The bladder is lost, and is of no 

 importance, but the " head " or scolex fixes itself to the 

 wall of the intestine. There it is copiously and richly 

 nourished, and buds off asexually a chain of joints. 



As these joints are pushed by younger interpolated buds 

 further and further from the head, they become sexually 

 mature, developing complex hermaphrodite reproductive 

 organs. The ova produced in these are fertilised, appar- 

 ently by spermatozoa from the same joints ; the proglottis 

 becomes distended with ripe eggs and developing embryos. 

 These ripe joints are liberated, and the vicious circle may 

 recommence. Happily, however, the chances are millions 

 to one against the embryo becoming an adult. 



The above history is true mutatis mutandis for many other 

 tapeworms. It will be observed that the embryo grows 

 into a proscolex or bladder, which buds off a scolex or head, 

 which, in another host^ buds off the chain oi proglottides, but 

 as it is virtually the same animal throughout, it is not 

 accurate to say that the life-history includes an " alternation 



