LIFE HISTORY OF T&NIA SOLIUM. 173 



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 proglottis ruptures, these egg 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 development, 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 de- 

 velopment 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 thirty- 

 five millions to one against the embryo becoming an adult. 



The above history is true mutatis mutandis for many other tape- 

 worms. 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 of proglottides, but as it is virtually the same animal through- 

 out, the life history does not include an "alternation of generations." 

 It is doubtful, however, what term should be applied to those cases in 

 which the bladderworm (Cccnurus and Echinococcus] forms not one head 



