DISEASES OF THE OX AND SHEEP. 453 



Ergot is, as we recently pointed out, parasitic upon various 

 grasses. The mistletoe is an evergreen plant, formerly held in 

 great veneratian by the ancient Druids, and growing upon apple 

 and oak trees. The Sarcina ventriculi, a vegetable organism 

 which takes up its abode within the human stomach, afifords an 

 instance of a vegetable being parasitic in the interior of an 

 animal. Moreover, there are different vegetals which grow upon 

 the skin, forming, for example, ringworm, while in the human 

 mouth we are confronted with the Leptothrix buccalis, which 

 has been said to be connected with the decay of teeth. Besides 

 ringworm, there are also other skin diseases, both in man and 

 animals, which are due to parasites. The Acarus scahiei pro- 

 duces itch in man, and other varieties of this arachnid afflict the 

 dog, the sheep, the ox, and other animals. 



This term ** parasitism " has not, we believe, been extended to 

 the many cases in which minute vegetable germs, or rather 

 organisms, find their way into the blood and lymph- vessels, and 

 the various tissues of different animals, by their vital activities 

 bringing on disease and death ; yet they are certainly very 

 similar to those in which life is carried on by very lowly developed 

 living things at the expense of the highest animals. 



There is also to be mentioned another group of examples. 

 Every species of animal is subject to the inroads of organ- 

 isms belonging to the great class Vermes, and a peculiar point 

 is that as a rule the worms which infest one animal are distinct 

 from those which invade other animals; though in certain cases 

 the same worm invades several, and even in some instances 

 many, different kinds of creatures. Another and still more 

 noteworthy feature is that many worms go through different 

 phases of development, and that these phases are passed through 

 in different animals. Indeed, the strangest of facts have been 

 elicited by the students of Helminthology, facts which at first 

 seem nothing less than startling. The life-history of these 

 creatures is, in short, one of the most interesting of all topics 

 of study. "Whether dealing with the external or internal 

 forms," writes the illustrious helminthologist, the late Dr. T. 

 Spencer Cobbold, ** the study of parasites of man and animals 

 is practically one of boundless extent; and there is probably no 

 department of knowledge, possessing an equal value in relation 

 to the welfare of man and beast, that is so thoroughly misunder- 



