OF MICRO-ORGANISMS. 49 



abounds, the collisions are frequent, their quest profit- 

 able, and sustenance easy; when scarce, the en- 

 counters are correspondingly less frequent, the ani- 

 mal fasts and keeps his Lent. The Lagynus crassicolis, 

 accordingly, never sees its victim from a distance and 

 in no case directs its movements more towards one 

 object of prey than towards another. It roams about 

 at random, now to the right and now to the left, im- 

 pelled merely by its predatory instinct — an instinct 

 developed by its peculiar organic construction, which 

 dooms it to this incessant vagrancy to satisfy the re- 

 quirements of alimentation. 



"The vortical Infusoria, when in a medium abound- 

 ing in food, are almost entirely sedentary in their 

 habits, only making slight changes of position. But 

 if they are placed in a medium affording but little nu- 

 tritive material, they become as migratory as the hunt- 

 ers, and are seen to race about in all directions search- 

 ing for more abundant nutriment. It is hard to find 

 a more perfect illustration of the influence exerted by 

 the conditions of a medium upon the habits and 

 customs of animals. 



"The Leucophrys patula is a type distinctively car- 

 nivorous and possessed of an extremely voracious ap- 

 petite, a fact which explains its power of multiplica- 

 tion, one of the greatest I have studied. With a tem- 

 perature of 25° in my laboratory I have recently seen 

 it separate by fission seven times in twenty-four hours, 

 that is to say, a single individual produces from itself 

 just one hundred and twenty eight others in that 

 time. In constant pursuit of its prey, it seizes its vic- 

 tims by the two stout vibratile lips with which its 

 mouth is armed, and swallows them alive and whole. 

 The victims may be seen struggling and tossing about 



