THE TREMATODA 



75 



This history is the best known, and is true only within 

 certain limits for the whole group ; for in some cases one genera- 

 tion the redia is omitted ; in other cases the sporocyst may 

 form by gemmation a second generation of sporocysts, within 

 which the cercariae arise. 



The sporocyst and redia are always parasitic in some mollusc ; 

 but the free-swimming cercaria chooses a great variety of hosts 

 in fact, nearly any invertebrate may serve, though it is not neces- 

 sary for it to become parasitic at all ; the adult, however, is found 

 in members of all classes of Vertebrata, rarely in Carnivora, and 

 nearly absent in Pigeons and certain other birds ; Trematodes are 

 .especially common in fishes, aquatic birds, reptiles, insectivorous 

 birds and mammals, and marine mammals. 



The cercariae are not all tailed, and this tail may present great 

 differences in size and structure. Many cercariae are known, some para- 

 sitic, others free-swimming, whose adult stage is unknown. And, as in the 

 case of the adult fluke, a species of cercaria may enter a variety of species of 

 host ; thus C. armata enters Paludina and Planorbis ; further, one host may 

 contain quite a number of different species of cercaria, e.g. L. stagnalis, 

 may harbour as many as eight species. Cercariae, with a well-developed 

 tail, are most numerous; the tail may be 

 provided with a fin-like membrane (G. lopho- 

 cerca\ or with bunches of " setae," regularly 

 arranged (G. setifera), (Fig. XVII.). It ac- 

 quires an enormous development in C. 

 macrocerca from Cyclas, and in C. elegant, 

 which occurs free in the sea. The tail is 

 retractile in G. mirabilis; it is bifurcated in 

 Bucephalus. The tail is quite rudiment- 

 ary in G. limacis and others, while the tail 

 is absent (" cercariaea ") in Leucochloridium 

 and others which live in terrestrial molluscs. 



There is thus produced in the life- 

 history of the Malacocotylea a consider- 

 able number of flukes from each egg 

 cell by the intervention at one or more 

 stages of some form of non-sexual re- 

 production ; these asexual forms live in 

 a host different from that of the sexual 

 fluke ; hence van Beneden gave the 

 name Digenea to the group. But 

 amongst the Digenea were included As- 

 pidogaster and the Holostomidae, which 

 do not agree with this general history. 

 Aspidogaster has, owing to the possession of a number of anatomical 

 peculiarities, been removed from the group. The Holostomidae 



1. 



FIG. XVII. 



Cercaria setifera, Villot, in a 

 state of extension ; the long tail is 

 provided with a series of circles of 

 stiff bristles. 



2. C. fissicauda, Villot. Both oc- 

 cur in the marine lamellibranch 

 Scrobicularia tennis. (After Villot.) 



