186 PROCEEDINGS OP THE AMERICAN ACADEMY. 



The most easily observed responses of animals are naturally those 

 which find their expression in locomotion. The number of stimuli which 

 may influence locomotion are, of course, numerous, but of these a certain 

 limited number play much the larger part. If we had an accurate 

 knowledge of the relative weight of these different forces, we might pre- 

 dict with certainty the path any animal would follow under certain given 

 conditions. An experimental study of the different stimuli ought at least 

 to enable us to find out which ones do operate, and perhaps to establish 

 certain general laws regarding them and the biological tendencies which 

 impel the animal to respond. 



The present paper is a study of the locomotor responses of the slug 

 Limax maximus to three kinds of stimuli, — those of touch, gravity, and 

 light. In connection with these studies new problems have constantly 

 arisen, some of which have been cursorily considered, many others 

 merely alluded to, so that the work is far from being complete. 



The term " geotaxis " has been used to designate the influence of gravity 

 on locomotion. Interesting and careful studies have been made on the 

 geotaxis of numerous Protista by Schwarz ('84), Aderhold ('88), 

 Massart ('91), and Jensen ('93). These investigations clearly show a 

 geotactic response in the unicellular organisms studied. The kind of 

 response varies according to other conditions, such as those of light, heat, 

 density of medium, chemical influences, etc., and may also differ in indi- 

 viduals of the same genus under apparently like conditions. Massart 

 ('91, pp. 161-162) found that, when a number of Spirilla were put into 

 a vertical tube, one group collected in the upper part and another at the 

 lower part. He also found (p. 164) that Chromulina woroniniana was 

 negatively geotactic — that is, moved upward, or in a direction opposite to 

 that of the pull of gravity — at 15° to 20° C, but positively geotactic at 

 5° to 7° C. Jensen's work also showed the important influence of other 

 agents in modifying geotaxis. Loeb ('88, pp. 7-8) found that cock- 

 roaches preferred the steepest side of a box whose four sides were inclined 

 at different angles ; that is, they are negatively geotactic. He also dis- 

 covered that a number of other Metazoa were geotactic. 



In a certain way, the present paper is a continuation of a recent study 

 made by Dr. C. B. Davenport and Miss Helen Perkins on geotaxis in 

 the slug. Davenport and Perkins ('97, p. 105) discovered that the 

 intensity of the animal's geotactic response was directly proportional to 

 the sine of the angle of deviation from the vertical, and hence " varied 

 directly as the active component of gravity." In the third section of 

 their paper, the question, " What determines whether the head end of 



