BEHAVIOR IN LOWER METAZOA 247 



character, yet are not recognized as such by the author, on the watch 

 only for "tropisms." In the earlier literature of animal behavior, be- 

 fore the prevalence of the recent hard-and-fast theories, one finds the 

 trial movements fully recognized and described in detail. This is the 

 case, for example, in the classical papers of Engelmann on behavior in 

 unicellular organisms, and, as we have seen in detail, for that of Preyer 

 on the starfish. Moebius, in 1873, gave a lecture on behavior in which 

 examples of this fact are found. Thus, he describes the reaction of a 

 large mollusk, Nassa, to chemical stimuli, as shown when a piece of 

 meat is placed in the aquarium containing them, in the following way : 

 They do not orient themselves in the lines of diffusion and travel toward 

 the meat, but move "now to the right, now to the left, like a blind man 

 who guides himself forward by trial with his stick. In this way they 

 discover whether they are coming nearer or going farther away from 

 the point from which the attractive stimulus arises" (Moebius, 1873, 

 p. 9). 



Unprejudiced observation of most invertebrates will show that they 

 perform many movements which have no fixed relation to sources of 

 external stimuli, but which do serve to test the surroundings and thus 

 to guide the animal. This the present author has observed, for example, 

 in studies on the leech, on various fresh-water annelids and mollusks, 

 and in less extended observation on many other animals. As Holmes 

 (1905) has recently pointed out, in a most excellent paper, this is really 

 a matter of common observation on all sorts of animals. The fact that 

 such movements are not emphasized by writers on animal behavior is 

 evidently due to their being considered without significance. 



In a number of recent papers the importance of trial movements in 

 behavior has been more explicitly recognized. Thus, for the earthworm, 

 the recent papers of Miss Smith (Mrs. Philip P. Calvert) (1902), of 

 Holmes (1905), and of Harper (1905) have set this matter in a clear light. 

 Miss Smith showed that in the reactions of the earthworm Allolobophora 

 fcetida to heat and cold, to chemicals, to drying, and to light, "testing 

 movements" play a large part. When stimulated, the earthworm fre- 

 quently responds by moving the head first in one direction, then in an- 

 other, often repeating these movements several times. It then finally 

 follows up those movements which decrease the stimulation. Holmes 

 (1905) confirms these results, especially for the reaction of the earth- 

 worm to light. His account of the behavior of the earthworm under 

 the action of light coming from one side may be quoted: "It soon de- 

 veloped that what seemed at first a forced orientation, the result of a 

 direct reflex response, is not really such, but that the orientation which 

 occurs and which is often quite definite is brought about in a more indi- 



