194 PROCEEDINGS OP THE AMERICAN ACADEMY. 



reveal that it has very different degrees of control over these two 

 regions of the body. In locomotion, the head end of the body, back 

 as far as the respiratory opening, is freely swung about from side to 

 side and determines the axis of orientation of the animal. Over the 

 posterior region, the animal seems ordinarily to have very imperfect 

 control. The relation between the two regions is crudely that of a 

 span of horses to a chain of wagons which they are pulling. When the 

 horses change direction, the wagons come only slowly around into posi- 

 tion one after the other, and there is likely to be some slipping in the 

 process, especially if it takes place on a down grade. In watching the 

 slug, I saw that the adhesion of the anterior region appeared consider- 

 ably greater than that of the posterior. When the animal gets dry, it 

 does so first at the posterior region. The tip of the tail is the part first 

 to lose its clinging power, and it may curl up dorsally as a result of the 

 drying process. If an animal which is thus beginning to deteriorate in 

 its supply of mucus be put on a glass plate and the plate raised into a 

 vertical position, the slug will move along and desperately cling to the 

 plate with the anterior part of its body. The posterior region will 

 gradually swing downward as a result of the pull of gravity, and, in 

 consequence, the animal's head will eventually be directed upward. 

 From this, we are justified in concluding that the same principle will 

 operate, although to a considerably, less degree, in the animal's normal 

 condition. A hasty examination showed that there was a good deal of 

 variation in the proportions of the two regions in different individuals. 

 As a crude and easy way of estimating these proportions, I measured 

 the length in millimeters of the anterior region from the tip of the head 

 to the posterior fold of the mantle, and similarly the length of the pos- 

 terior region from that fold to the tip of the tail.* 



The results from 27 animals thus measured are given in Table IV. 



The individuals (Table IV.) are arranged in a series, beginning with 

 those in which the two regions are most nearly of the same length 

 and ending with those in which the disproportion is greatest. In animal 

 No. 1, the length of the anterior region is 83.3 per cent (column 8) of 

 the posterior ; that is, the ratio is almost one to one. In No. 25, the 

 anterior region is only 45 per cent as long as the posterior, or less than 

 half its length. 



The fifth column in the table gives the geotaxis of individuals in per 



* The measurements were made when the animal was extended and moving 

 across the plate. The amount of elongation varies a good deal, but the regions 

 retain pretty closely their relative proportions. 



