286 PAEASITISM. IN PLANTS AND ANIMALS. 



on an inactive condition, attach themselves to blades of grass, 

 and become " encysted" or covered with a dense external envel- 

 ope. Here again they fail to complete the cycle of their exist- 

 ence unless they be swallowed along with their blade of grass 

 by some browsing sheep. Should this occur they again become 

 active, bursting through the tough outer coat of the cyst, and 

 perforating the sheep's tissues are carried by the blood current 

 into the liver, where they undergo their final metamorphosis 

 into the adult fluke. It is clear, then, that at every stage of 

 this parasite's existence there are many chances against its find- 

 ing conditions suited to its needs, and were it not for the enor- 

 mous number of ova produced — not only by the fluke but by all 

 parasitic worms — the species would have little chance of survi- 

 val. The ordinary tape-worm of man is stated to live two years 

 and to produce 85,000,000 ova. It is certain that only one ovum 

 out of many thousands passes through the ordeal to its final 

 stage of development. The study of this life-history shews that 

 the cure for " rot" in sheep must be sought in the prevention of 

 the disease, and that this can certainly be attained only by keep- 

 ing the animals in dry pastures where the "intermediate host," 

 necessary for the growth of the fluke, cannot exist. 



I have already mentioned the degradation of structure which 

 is observed in vegetable parasites : this is even more remarkable 

 amongst animals. Some of the most extreme cases are met with 

 amongst the small Crustaceans known as fish-lice. These crea- 

 tures are found, often in considerable numbers, attached to the 

 skin, gills, eyes, or other organs of fishes, to which in many cases 

 they adhere, not merely by suckers but by root-like growths, 

 which are inserted deeply into the body of the host and by means 

 of which its nutritive juices are absorbed. In this adult form 

 the animal may be little more than a mere misshapen bag of eggs 

 or spermatozoids, and it would be impossible, except by a study 

 of its development, to determine its true place in nature. But 

 the first days of its existence, after emergence from the egg, are 

 spent very actively : in that phase it is provided with several 

 pairs of swimming feet, and it is only after attaching itself to 

 the body of its future host that it loses its limbs and its activity, 



