xii TRANSLATOR'S PREFACE. 



nified development of its parent. From this point of view we may 

 compare the cystic worm to the caterpillar, or chrysalis stage in the 

 development of the insect. It differs, however, in this, that in 

 many cases the cystic worm has the power of developing, at this 

 stage of its growth, a large number of creatures resembling itself, 

 and these have, each of them, the power of developing themselves 

 into tape-worms, as though the caterpillar had the power of pro- 

 ducing in its interior any number of young caterpillars, each of 

 which would grow into a perfect butterfly. 



It is here that the history of our Entozoa becomes interesting 

 from another point of view. They are illustrations of Steenstrup' s 

 theory of the " Alternation of Generations." The cystic-worm 

 let it be an Echinococcus has originated from the egg of a tape- 

 worm, the embryo of which has found its way from the stomach 

 and intestines through their walls, into the tissues of the body. 

 This worm consists of a vesicle, or bag, to which is attached a 

 head, called the " scolex." 1 In Cysticercus, the hydatid of the 

 pig, there is but one scolex, but in Echinococcus there are many 

 scolices. Now this scolex, or scolex-head, as it is sometimes 

 called, is the stock or germ the head from which all the 

 segments of a tape- worm proceed. The cyst of Echinococcus, 

 then, has the power of producing a large number of these heads, 

 each of which may grow into a tape-worm. The cyst, the 

 original cyst of the worm, is, in the language of Steenstrup, " a 

 nurse." Kiichenmeister and the Germans call it a mother-cyst. 

 But this cyst will not only produce scolex-heads, but other cysts 

 like itself. These are " daughter-cysts," and these secondary 

 cysts will also produce scolex-heads. They are also " nurses " 

 and in virtue of their existence the mother-cyst becomes, in the 

 language of Steenstrup, a " parent-nurse." The second cyst 



1 This word " scolex" was originally employed by Miiller to designate generically some 

 imperfectly developed forms of tape-worm. The head was the most characteristic part 

 of these creatures, and gradually the term "scolex" was applied to the heads of all 

 forms of cystic and tape-worms. The term scolex has now no generic signification, as 

 the creatures to which it was applied were immature forms of other genera. 



