1 86 BEHAVIOR OF THE LOWER ORGANISMS 



attached to each other, and sometimes forming chains of four or five. 

 But they never succeed in swallowing one another. They often try 

 rotifers in the same way, but the outer integument of these organisms 

 is so tough that Didinium does not succeed in piercing it, and the rotifer 

 escapes. Stentor and Spirostomum are often fastened upon, but usually 

 escape, owing to their large .size, great activity, and rather tough outer 

 covering. The reason why Paramecium is usually employed as food 

 rather than other organisms is clearly due to the fact that when the 

 Didinia try these, they usually succeed in piercing and swallowing them, 

 while with most other objects they fail. 1 



Didinium is a type of the hunter ciliates in this respect. The process 

 of food-getting is throughout these species one of trial of all sorts of things. 

 There is no evidence that in some unknown way the infusoria perceive 

 their prey at a distance, nor that they decide beforehand to attack certain 

 objects and leave others unattacked. They simply "prove all things 

 and hold fast to that which is good." 



We cannot do better in emphasizing this point than to quote a por- 

 tion of the words of the veteran investigator Maupas, as given in Binet's 

 "The Psychic Life of Micro-Organisms" (pp. 48, 49): 



"These hunter infusoria are constantly running about in search of 

 prey; but this constant pursuit is not directed toward any one object 

 more than another. They move rapidly hither and thither, changing 

 their direction every moment, with the part of the body bearing the bat- 

 tery of trichocysts held in advance. When chance has brought them in 

 contact with a victim, they let fly their darts 2 and crush it ; at this point 

 of the action they go through certain manoeuvres that are prompted 

 by a guiding will. It very seldom happens that the shattered victim 

 remains motionless after direct collision with the mouth of its assailant. 

 The hunter, accordingly, slowly makes his way about the scene of action, 

 turning both right and left in search of his lifeless prey. This search 

 lasts a minute at the most, after which, if not successful in finding his 

 victim, he starts off once more to the chase and resumes his irregular 

 and roving course. These hunters have, in my opinion, no sensory 

 organ whereby they are enabled to determine the presence of prey at a 

 distance; it is only by unceasing and untiring peregrinations both day 



1 Balbiani (1873) described Didinium as discharging trichocysts from the mouth 

 region against its prey, thus bringing it down from a distance. This account has not 

 been confirmed by other observers, and the writer has never seen anything of the sort in 

 the innumerable cases of food-taking in Didinium which he has observed. It can hardly 

 be doubted that the trichocysts represented in Balbiani's figure (our Fig. 113) really come 

 from the injured Paramecium, and not from the Didinium. 



3 This use of the trichocysts has not been confirmed by other writers and was not 

 absolutely observed by Maupas himself. 



