444 CHARACTERS OF INVERTEBRATE ANIMALS 



The internal structure of the Fluke is extremely complex, 

 and only a few of the more salient features can be mentioned. 

 The body-wall consists of the cuticle with underlying epidermis 

 and muscular layers, within which the spaces between the various 

 organs is filled up by a soft packing material, much as in a tape- 

 worm. Here, however, though special circulatory and respiratory 

 organs are absent, the digestive system is well developed, for the 

 surrounding food, consisting of blood and broken - down liver 

 substance, requires to be digested. The mouth leads into a 

 muscular pharynx, acting as a suction-pump, this into an extremely 

 short gullet, and this again into a forkecV intestine, the halves of 

 which are much branched and end blindly. The excretory organs 

 are much of the same kind as in the Tape- Worm, and the nervous 

 system is not much better developed. It consists of a ring round 

 the front part of the gut, thickened above into ill-defined cerebral 

 ganglia, and giving off a number of nerves, of which the most 

 important take a backward direction. 



The life-history is more complex than that of the Tape-Worm, 

 and of a remarkable character, including a number of stages. The 

 ripe eggs produced by the adult Fluke may be taken as the point 

 of departure. These pass down into the intestine of the sheep 

 and thence to the exterior. If they happen to fall into water or 

 on to damp grass, and fluke-disease is commonest in wet fields, an 

 elongated ciliated embryo hatches out from each of them. This 

 swims actively about for some time, and if it happens to come 

 across a certain small water-snail (Limncea truncatula], further 

 stages in its life-history become possible. The larva makes its way 

 into the lung of the snail and becomes a shapeless bag (sporocyst} 

 within which are developed a number of cylindrical rediae, con- 

 stituting the next stage. The redia makes its way out of the 

 sporocyst and travels to the liver of the snail, upon which it preys. 

 The last stage in the life-history is the tadpole-like cercaria, which 

 is really an immature fluke, and this is produced within the redia 

 much as that took origin within the sporocyst. The cercaria 

 leaves the snail, and, swimming through the water by means of 

 its tail, reaches a piece of grass or some other plant, to which it 

 becomes attached. The tail is lost and the body becomes invested 

 in a firm limy coat or cyst. If now a sheep should happen to 

 swallow one of these encysted forms, the limy covering is dissolved 

 by the action of the gastric juice, and the young fluke, escaping, 



