76 THE TREMATODA 



present this difference from the rest of the Malacocotylea ; the 

 young form, after entering the intermediate host, does not reproduce 

 asexually, but develops by a metamorphosis into the adult form, 

 when the intermediate host is swallowed by the final host ; Leuckart 

 has used the term " metastatic " in reference to this life-history. 



This life-history of the common fluke is a favourite example 

 of " alternation of generations," or metagenesis, on the view that 

 the mode of reproduction in sporocyst and redia is an asexual one 

 (viz. budding) ; but Grobben first suggested that the cells which 

 give rise to " germ balls " are essentially ova, and that it is a case 

 of parthenogenesis a view with which Leuckart essentially agrees. 

 In that case, the process is one of " heterogamy " (Leuckart) ; but 

 this term is more generally applied to cases in which two sexual 

 methods alternate, as in PJiabdonema nigrovenosum ; and Schwarze 

 has invented the term " alloiogenesis " to indicate alternation of 

 parthenogenesis with sexual reproduction. Glaus, regarding the 

 redia and sporocyst as larvae, sees in the history an example of 

 heterogamy with paedogenesis. 



Leuckart, Balfour, and Looss regard the whole process as one 

 of a metamorphosis distributed over two or more stages (or gene- 

 rations), as a result of the appearance of vertebrates on earth ; for 

 before their appearance the flukes must have attained maturity in 

 an invertebrate, which, on the evolution of vertebrates, became 

 the intermediate host. Looss (30) has further shown that the sporo- 

 cyst, redia, and cercaria are all built upon a common plan, and 

 represent successive stages in development, the last being entirely 

 fluke-like, except for the full development of the generative organs. 



The life-history of D. macrostomum is of interest in that the 

 redia and the free-living stage of the miracidium and cercaria are 

 omitted (see 20). The fluke inhabits the intestine of various sing- 

 ing birds, and its eggs pass out with the faeces of its host, which, 

 falling on a leaf, may with it be eaten by the gastropod Succineaputris 

 (Fig. XVIII.). In the stomach of the snail the miracidium, of 

 peculiar form, is hatched out, and makes its way through the wall of 

 the intestine into the connective tissue. Here it becomes a sporo- 

 cyst; this grows very rapidly, absorbing the blood of the host, and 

 gives rise to numerous branches, one or two of which outrun the 

 rest, and push their way into the snail's tentacles. The branches 

 become banded with olive-green or brown, and the structure is now 

 known as Leucochloridium paradoxum (discovered by Carus, 1835). 

 Owing to its colouring and pulsations within the tentacle, it is 

 mistaken by birds for a dipterous larva, and is devoured. The 

 " cercariaea," which have in the meantime developed from the germ 

 balls inside the sporocyst, are without tails, which are evidently 

 not required, as the organism never leads an independent free 

 life ; they develop in the bird into a fluke. 



