118 



LIFE-HISTORY OF PAKASITES. 



existence of sexual generation ; in fact, it may in a sense be regarded 

 as the last trace of this process, especially as the germ-cells possess 

 an unmistakeable morphological similarity to ova. 1 The importance 

 which this alternation of generations has for the preservation and dis- 

 tribution of these parasites is evident. Where formerly there was 

 only one parasite there will now originate a number many dozens, 

 or perhaps even more all readily capable, under favourable con- 

 ditions, of commencing new parasitic life. 2 



The newly formed Distoniidae, however, do not grow into sexually 

 mature animals within or beside their parents, but, as a rule at least, 



FIG. 75. Redise, with brood of Distomes in the interior. (A.) From Paludina 

 impura (young and old) ; (B.) From Lymnceus (young and old). 



forsake the host as a Cercaria, and swim about in the free state for a 

 time by means of an appendage which is not unlike the caudal bladder 

 of Archigetcs, and then migrate into a new host, generally once more 

 an invertebrate animal (p. 72). The Cercaria thus undergoes a change 

 of host, which does not immediately transfer it to a vertebrate, as is 

 usually the case, but at first to an invertebrate again, such as a snail or a 

 water insect. In the present Distomida?, also, these two hosts are both 



1 This conception receives a new confirmation from the life-history of A ttantonema, 

 alluded to above (p. 98). 



2 Such a proliferation in the intermediate host we find in a few Cestodes, and especi- 

 ally in Echinococcus; but in this case the young brood originates through budding, and 

 remains conn- cted with its mother-animal in the interior of the body for life. 



