PARASITES AND THEIR DEVELOPMENT. 



553 



but powerful beyond comprehension in virtue of 

 their numbers, devastate the crops which exercise 

 his mind and appliances in their cultivation. And 

 after the crops have been duly stored and gar- 

 nered, the labor of months and the full fruition 

 of the farmer's hopes may be destroyed by the 

 insidious attack of granary - pests. Plants of 

 lowly grade — minute fungi and like organisms — 

 personally known to the microscopist alone, blight 

 at once the prospects of the agriculturist and of 

 his cereals. A minute fungus, burrowing its way 

 within the tissues of the potato - plant, has ere 

 now brought destitution and famine on a nation, 

 and still causes disease among our tubers to an 

 extent which none but our potato-growers can 

 fully realize. Nor is the farmer's sphere singular 

 in respect of its liability to the attack of animal 

 and plant foes. Parasites, the complexity of 

 whose life-history almost defies belief, invade the 

 stock of the breeder of cattle and sheep and 

 decimate his flocks; while these same parasites 

 may occasionally invade the human domain itself, 

 and cause disease and death to prevail to an 

 alarming extent. Hidden enemies in the sea bur- 

 row into the sides of ships, or undermine man's 

 piers and bulwarks. Poison-traps lie in wait for 

 human footsteps ; and claw and tooth are as ruth- 

 less when opposed to humanity as when prepared 

 to attack lower life. Speaking generally, there- 

 fore, man may be readily shown to be by no 

 means the undisputed "monarch of all he sur- 

 veys " in the territory of either botanist or zoolo- 

 gist ; and the province of mind and intellect may 

 be invaded by foes against which man may find it 

 impossible to contend. Much has been done, it 

 is true, in the way of repressing many of our 

 lower enemies, and the increase of scientific 

 knowledge has had few triumphs of higher kind 

 than are witnessed in those researches which 

 have exposed the nature of our animal and plant 

 enemies, and shown us the steps necessary to be 

 taken for their annihilation. But the field of in- 

 quiry seems wellnigh boundless ; and it should 

 certainly form one of the most powerful argu- 

 ments in favor of the study of natural science, 

 that on the advance in our knowledge of eco- 

 nomic botany and zoology the prosperity of our 

 commerce and the conservation of our health 

 may be shown largely to depend. 



Perhaps one of the richest fields of research 

 in the way of repression of our lower enemies is 

 offered by the life-history of some of the most 

 common parasites which decimate our flocks and 

 herds, and which, as already remarked, occasion- 

 ally invade the human territory itself. Well does 



the shepherd know the symptoms of "rot" in 

 his flock, and anxiously does he apply to the 

 veterinarian for advice in his extremity. Ilis 

 sheep, in such a strait, present a dull and dejected 

 appearance ; they are "off their feed," he will tell 

 the observer, and are in a thoroughly emaciated 

 condition, despite the shepherd's kindly care and 

 supervision. By-and-by deaths will begin to be 

 of frequent occurrence, and when the dead sub- 

 jects are carefully inspected the cause of the dis- 

 order is not hard to discover. The body of the 

 affected sheep exhibits a state of thorough dis- 

 organization, and, when the liver is carefully in- 

 spected, hundreds of small, flattened bodies, each 

 about three-quarters of an inch long, are found 

 within the bile -ducts; while in the bile itself 

 thousands of small particles are to be discovered 

 by microscopic aid. The small, flat bodies are 

 "flukes," and the particles are the eggs of these 

 animals. What, it may be asked, are these 

 "flukes" which, according to trustworthy evi- 

 dence, carry off annually between one and two 

 millions of sheep at the very lowest computation ? 

 The reply to this question is readily given. The 

 "liver-fluke" is one of a group of internal para- 

 sites which has been known from comparatively 

 early times. It was certainly known in 154*7, 

 and was lucidly described in 1552 by an author 

 who was shrewd enough to attribute to its pres- 

 ence an epidemic which decimated the flocks of 

 Dutch farmers in that year. Its " area of distri- 

 bution," to use a scientific but expressive phrase, 

 is not confined to sheep alone, but includes cattle, 

 the horse, hares and rabbits, the spaniel, deer and 

 antelopes, and even man himself. A little flat and 

 somewhat oval body, with a tree-like arrangement 

 of tubes for a digestive system, and possessing a 

 couple of suckers for adhesive purposes — such 

 are the main features which a "liver-fluke" pre- 

 sents for examination. A more innocent-looking 

 animal could hardly be found, and the cause of 

 its injurious effects upon its animal hosts might 

 remain a mystery, did our inquiries cease with 

 the investigation, so to speak, of its personnel. 



A highly-important consideration, however, 

 and one which extends beyond the restricted do- 

 main of our present subject, is that which recog- 

 nizes in numbers and time two important factors 

 in elevating agencies of apparently unimportant 

 kind into forces of vast or uncontrollable nature. 

 The rain-drop is insignificant regarded merely as 

 a particle of water, no doubt ; but multiply your 

 rain-drops indefinitely, and you obtain the agent 

 which will wear the hardest rock, excavate the 

 giant-cavern, or form the foaming cataract with 



