peesident's addkess. 207 



warm-blooded vertebrate, in whose alimentary canal it undergoes 

 its final development and becomes sexually mature. The young 

 are produced viviparously, perforating the coats of the stomach 

 and intestines, and ultimately reaching the muscles, these pro- 

 cesses giving rise to a train of symptoms closely resembling 

 those of rheumatic fever, — more or less severe according to the 

 niimber of the parasites, and not unfrequently fatal in their 

 results. The length of the male Trichina is one-eighteenth of 

 an inch, of the female one-eighth of an inch, and each mother 

 is probably capable of producing from ten to fifteen thousand 

 young. It is stated that an ounce of pig's flesh may contain as 

 many as eighty thousand TrichincB, and one pound of siich flesh 

 might, according to Dr. Cobbold, produce a crop of four hundred 

 millions, l^o wonder that the migration of such an invading 

 host is liable to be attended with disastrous results. 

 " "With one other instance of transmigration, — more wonderful 

 than those of Indur — I will leave the consideration of the larger 

 Entozoa. The disease called "rot" in sheep arises fi-om the 

 presence of a flat trematode worm known as the liver-fluke, a 

 parasite which is found sometimes, though rarely, in the human 

 organism. The migrations of this animal during development 

 are perhaps even more astonishing than those which have just 

 occupied our attention. The adult fluke — Distoma hepaticum — 

 lives in the liver and gall-ducts of sheep which feed in damp 

 or marshy pastures : in dry, upland pastures it is unknown. It 

 is an ovate, leaf -like animal, about half an inch to an inch or 

 more in length, with a distinct mouth and two suctorial discs, 

 by means of which it attaches itself to the interior of the bile- 

 ducts. It produces numerous ova — ovate bodies with a long 

 diameter of about -rr^u-th of an inch — which pass into the intes- 

 tine and are expelled with the contents of that canal. The ova 

 cannot hatch except in water, but if this is reached they at once 

 give rise to small embryos which, being ciliated, swim about 

 freely for a few hours, ultimately dying unless they find access 

 to the body of the intermediate bearer, Limncea truncatula, a 

 common fresh-water snail. Attaching itself to the Limnma, the 

 embryo bores into the tissues of its host, and enters the body 



