146 



Platyhelminths. 



which in some forms give off more dehcate branches (Fig- 102)^ in 

 others are uubranched (Fig. 95) . The genital openings are 

 usually on the ventral side close together, and far forward. The 

 Trematodes are invariably hermaphrodite, bat in some {see below) 

 female parthenogenetic generations occur. 



The ectoparasitic forms pass through a 

 ciliated larval stage, during which they 

 swim freely in the water. The reproduc- 

 tion of most of the endoparasites is 

 far more complicated. The different gene- 

 rations alternate with one another, a her- 

 maphrodite being followed by one or more 

 parthenogenetic generations, so that a 

 heterogony occurs. A ciliated larva 

 hatches from the fertilised egg of the 

 hermaphrodite generation, migrates into a 

 lower animal, generally a Gastropod, and 

 there develops into an imperfect female, 

 within whose body the eggs are formed; 

 an alimentary canal is wanting or occurs as 

 a simple sac. The eggs produced by this 

 generation develop without fertilisation in 

 the body of the parent, and these individuals, 

 in some forms, constitute the hermaphro- 

 dite generation, after they have been received into another host (a 

 Vertebrate) . In others, however, they form a second female (partheno- 

 genetic) generation, which then produces the hermaphrodites. These 

 are always far superior to the others in size and in complexity of 

 organisation, and in many forms they alone are known. 



Fig. 1 02 . Sketch of Liver- 

 fluke, showing alimentary 

 canal X 2. Si anterior, s^ 

 posterior sucker, ta gut. — 

 After Thomas. 



Order 1. PolystomeSB {Monoqenetic Trematodes). 



Almost always ectoparasitic : with usually more than two suckers, 

 often possessing hooks : without heterogony. The majority are 

 parasitic upon Fish (skin and gills) . 



1. Tte genus Tristomvm, includes large species (to almost 2 c/m long) ; 

 characterised by a very large sucker at the Mnd end, and two smaller ones 

 anteriorly ; they are parasitic on many marine Fish. 



2. Polystomum mtegerrimum (Fig. 105), lives in the urinary bladder of the 

 Frog ; anteriorly there are four eyes ; posteriorly six large suckers, and a number 

 of hooks of different sizes. The eggs are laid in the spring, pass to the exterior 

 through the cloacal aperture of the host, and develop in the water in the course 

 of some weeks. The larva is furnished with a ciliated girdle, with eyes, and with 

 a circle of sixteen hooks upon a disc at the posterior end ; the sucker of the adult 

 is absent. The larva wanders into the gill cavity of the tadpole, where it loses the 

 cilia and develops one or two pairs of suckers ; it remains there until the gills of 



