196 PROCEEDINGS OP THE AMERICAN ACADEMY. 



cents. The table includes those animals which were fairly active in 

 response but does not give individuals obviously unable to respond 

 because of a lack of slime secretion. The positively geotactic animals, 

 with two exceptions, are all found in the upper half of the table and 

 almost all the negative animals in the lower half. Supposing other 

 conditions the same, we can say that those animals in which the ratio 

 of anterior to posterior regions is as 2 : 3, or greater, will be positively 

 geotactic. Those between the ratios of 2 : 3 and 3 : 5 will be more 

 uncertain in their geotaxis, which will depend largely on the combina- 

 tion of other conditions. Finally, those in which the ratio is less than 

 3 : 5 will almost invariably be negatively geotactic. The nearer one 

 gets to the extremes, the greater the accuracy of prediction. This pre- 

 diction, it is understood, applies only to animals tested on the glass plate. 



An examination of the ninth column shows that the few cases of nega- 

 tive geotaxis occurring in the positive half of the table are probably due 

 to a deficiency in the second most important factor affecting the geotaxis ; 

 namely, the condition of the slime secretion of the animal. This 

 secretion may be deficient either (1) in quantity, as in the case of slug 

 2 b ; or (2) in quality, as was the case with slug 10. Of the two cases 

 of positive geotaxis occurring in the negative half of the table, the first, 

 that of slug 18, is easily explained as due to an extraordinary tenacity 

 of the mucus. Moreover in this, and more markedly in the case of 

 slug 22 b, the slugs were very large and rather slow in their movements. 

 Slug 22 b, instead of moving ahead actively, like most slugs when in 

 good condition, often swung its head toward the earth without any fore- 

 ward movement, and hence did not give the pull of gravity the most 

 favorable opportunity to work on the posterior region of its body. This 

 connects itself with a general observation on all the animals. When 

 active, they are usually very precise and uniform in their responses. 

 If stupid, slow, and averse to movement, — a condition in which the 

 best of them sometimes get, — they will either obstinately refuse to 

 move, or else, keeping the posterior region firmly fixed, will swing the 

 head end toward the earth. Sometimes such a slug will slowly move 

 in a circle, first down then up, and finally curl itself up, like a dog by 

 the fireplace, and apparently go to sleep. This peculiarity may be 

 connected with the food conditions of the animals, as will be shown in 

 a set of experiments to be given later on. 



The two most important factors in determining the geotaxis of indi- 

 vidual slugs are, therefore : first, the proportion of the anterior (mantle- 

 covered) and posterior (uncovered) regions of the body ; secondly, the 



