14 Trans. Acad. Sci. of St. Louis. 



Sakcoptidae. — A very Important family composed of the itch mites. 

 They live in the skiu, and in a £e'«v' cases in the internal organs of 

 other animals. Here degeneration reaches its limit. 



Cttoleichidae. — Only two species known. They live in or upon the 

 skins of fowls, or in the air passages of the same. 



Demodecidae. — Includes but one genus, Demodex, which occurs in the 

 hair follicles of mammals. The body is vermiform and the legs 

 are reduced to stumps. 



Eriophyidae. — Consists of several genera of vermiform Acarlna which 

 live upon plants. Most of them produce galls or other malforma- 

 tions of leaves. 



Opilioacaf.idae. — Habits unknov.n and systematic position uncertain. 



Paths of Evolution from the Feee-livikg to the Para- 

 sitic Forms. 



In our study of the origin of parasitic forms from 

 free-living types two facts appear pre-eminently impor- 

 tant; first, no great or sudden change in the nature of 

 the food must be encountered; and, second, there should 

 be but a gradual change in the environmental conditions. 



A study of the way in which parasitism has arisen in 

 many of the larger natural groups of the animal king- 

 dom, has shown us that it has followed at least several 

 well defined courses. Of these several lines of descent, 

 as we may call them, the first of which I will treat, and 

 which is the simplest of all, is the origin of parasitism 

 in predaceous groups of animals. In such cases it is 

 evident that often there would be little and frequently 

 no change whatever in the nature of the food, in the 

 transition from the predaceous to the parasitic life. 

 Wliat is yet more significant of the gradual transition 

 from the predaceous habit, is that forms now exist that 

 are actually on the border line between the two and 

 might equally well be called predaceous or parasitic, ac- 

 cording as to whether they attack forms smaller or larger 

 than themselves. This point might be illustrated in the 

 case of some of the leeches, which may attack forms 

 larger than themselves and thus be parasitic, or attack 

 form-s smaller than themselves, in which case they might 

 be called predaceous. The same might be said of some 



