20 



TA EPIZOA. 



By J. McGregor Baxter, M. D. 



The term epizoa is derived from two Greek words epi, upon, 

 and zoon, an animal, and is used to represent comparatively min- 

 ute animals that prey on other larger animals, such as fish, 

 beetles, insects of all kinds, birds, quadrupeds and even man 

 himself, as almost every animal has its own particular parasitic 

 enemy or enemies. 



These animals are either true parasites, or are parasitic at 

 certain stages of existence. I think it is generally believed that 

 originally all of these animals led a separate and independent 

 existence, and only by accident, or by stress of circumstances, 

 were led to adopt a true parasitic life, for, if I recollect rightly, 

 I have read somewhere that they all show diminutive and atro- 

 phied organs, such as wings, etc., that show they really are degen- 

 erated oft'spring of animals that originally led an independent ex- 

 istence ; and indeed this transformation may be partial or al- 

 most complete, as the departure from the original natural mode 

 of life is more or less complete. 



We find this feature scattered throughout all nature, that 

 organs that are not used nor needed dwindle away till at last 

 they become merely rudimentary, as, for example, birds and in- 

 sects that inhabit islands in the ocean gradually lose their wings, 

 or at least they become so attenuated as to be useless for the 

 purpose for which they were originally intended. 



Let us take, for example, an independent living animal and 

 suppose that by accident, or the dearth of food, it is induced first 

 to attach itself to a larger animal wandering in its vicinity. It finds 

 that by biting or scratching this animal it can derive ready-made 

 and assimilated food at little expended effort on its own part,and 

 you have the initial stage. Then, developed b\' use, natural se- 

 lection and the survival of the fittest in the battle for life, you 

 can easily suppose that the organs of such an animal during 

 many generations would become more and more adapted to its 

 selected habit, until you would get the i)arasite pure and simple, 

 with all organs not absolutely necessary nor much used, dimin- 

 ished or entirely wanting. In fact, we can find animals in all 



