94 ANIMAL METAMORPHOSIS. 



its head to the mucous membrane of the intestine, and develops 

 to its perfect form, consisting of a minute rounded head (Fig. lo), 

 with organs of attachment, which are a crown of hooks and 

 suckers, followed by a long-jointed, tape-like body (Fig. ii), which 

 is produced by a process of budding from the head, each joint 

 containing both male and female reproductive organs, and pro- 

 ducing innumerable eggs, which, under favourable circumstances, 

 again go through the life-circle we have just been considering. 



The Trematodes, or Flukes — another order of parasitic worms 

 — also undergo transformations as various and strange as those of 

 their relatives, the tape-worms. As an illustration, we may take the 

 Fasciola hepatica, or Liver-Fluke, which in its adult form is a flat, 

 lozenge-shaped organism, about an inch long and half-an-inch 

 broad, furnished with two suckers, inhabiting the gall-bladder or 

 biliary ducts of sheep, and giving rise to the disease known as the 

 "rot." When a sheep is affected with flukes, the eggs, to the number 

 of several hundred thousands, pass with the bile into the intestines, 

 and so on to the fields. Under suitable circumstances, these eggs 

 are hatched, producing a minute, conical, ciliated embryo, which 

 swims about until it comes into contact with a special water-snail 

 (the Liimiceiis trunculatus)^ into which, and which alone, it bores 

 its way till it gets into the pulmonary chamber or body-cavity, 

 when it undergoes metamorphosis. It loses its external layer of 

 ciliated cells, changes from the conical to the eUiptical form, and 

 becomes a sporocyst, or mere brood-sac, in which the next genera- 

 tion is produced. This next generation — the members of which 

 are called " redise" — burst through the wall of the sporocyst, and 

 migrate to other parts of the snail, especially the liver, on which 

 they feed. The adult rediae may develop daughter rediae for one, 

 two, three, or even four generations, but ultimately each redia 

 produces in itself about a score of germs, which develop into the 

 next stage, called " cercarise." The free cercaria has an oval 

 body, about one-thirtieth of an inch long, provided with two 

 suckers, the anterior part covered with spines. Two lobed, lateral 

 masses extend the whole length of the body, on each side of the 

 middle line. It has a tail, and is exceedingly active, but soon 

 comes to rest, and encysts itself on surrounding objects, when it is 

 taken up by the sheep as it grazes on the damp roots of the grass, 

 and in them ultimately reaches its stage of maturity. 



