v FASCIOLA 165 



is present on each side a flame-cell (Figs. 74, B, n; and 75, B). Con- 

 spicuous amongst the parenchyma which fills the interior are numerous 

 reproductive cells the germ-cells. 



The miracidium swims actively hither and thither but normally dies 

 after a short life of about eight hours unless it comes across a small 

 water-snail of a species (Limnaea truncatuld) common in marshy dis- 

 tricts. The miracidium is able to detect the presence of a snail in its 

 neighbourhood and is indeed able to follow up its track on the mud. 

 It approaches the snail and attaches itself to its soft skin by its 

 proboscis, a Limnaea which has been attacked by many miracidia being 

 given a furry appearance through the miracidia hanging on to it by 

 their probosces. In such a case the snail may be killed, but more 

 usually it is attacked only by one or a few miracidia and does not 

 suffer serious damage. The miracidium after attaching itself bores 

 through the skin of the snail, makes its way into the body-cavity and 

 eventually takes up its position in the large blood spaces in the roof 

 of the lung. Here it loses its shape, the nerve-ganglion and eye de- 

 generate and it becomes a mere bag (sporocyst Fig. 74, C) containing 

 the germ-cells. The latter undergo repeated division, giving rise to large 

 masses (g.c) at first rounded in form but gradually becoming elongated. 

 Each of these gradually takes on the form of the next stage of the life- 

 history, known as the redia (Fig. 74, D) a worm-like creature with two 

 short stumps projecting from its body one on each side towards its hinder 

 end. At the front end is the mouth which leads through a muscular 

 pharynx into a short simple blindly-ending intestine (ent). The interior 

 of the body is filled as before with parenchyma containing scattered masses 

 of germ-cells (g.c) and near the head end is a single reproductive opening 

 (b.o). The rediae when fully formed make their way out of the remains 

 of the sporocyst and wander through the tissues of the snail where they 

 are found especially in the liver. The germ-cells within the redia com- 

 monly develop into a new generation of rediae, but the germ-cells 

 of this second generation develop not into rediae but into a type of 

 larva known as a cercaria (Fig. 74, E, c, and F). The cercaria is a 

 somewhat tadpole-shaped creature with an ellipsoidal body and a tail, 

 by the flapping movement of which the cercaria is able to swim. There 

 is a mouth at the front end, surrounded by a sucker and leading into a 

 pharynx which opens into the intestine of characteristic n -shape (Fig. 

 74, F, ent}. A ganglionic nerve-ring (g) surrounds the alimentary canal 

 in the region of the pharynx. Towards each side of the body is a mass 

 of gland-cells (gt). The space between these organs is filled with paren- 

 chyma. Finally about the middle of the body is a second sucker the 



