EVOLUTION 115 



to the successive raising, moving forward, and re- 

 planting down of succeeding portions of the under 

 surface of the foot, and is comparable to the mode 

 of progression in a caterpillar, save that only the 

 creeping surface and not the whole body participates 

 in the action. The lateral margins frequently do 

 not share in this motion, but have a gentle, lateral, 

 undulating movement of their own. 



Progressing in this way it has been calculated that 

 an average-sized Snail of moderate pace travels at 

 the rate of about a mile in sixteen days fourteen 

 hours, while a Slug, having no " house " to carry, can 

 cover the distance in eight days. 



This mode of locomotion is assisted by the secre- 

 tion, from glands in the foot, of mucus, which acts as 

 a lubricant. The principal of these glands are 

 situated at the front of the foot, in the middle of its 

 under surface, and at its extremity. The mucus in 

 many cases solidifies on contact with the air or water, 

 as in the familiar shining track of the Snail. In 

 certain of the Slugs it is sufficiently abundant and 

 tenacious to permit of the animal utilizing a thread 

 of it to climb from one leaf to another, or to descend 

 from an elevated object, and even to reascend by 

 climbing the filament thus formed. The Water 

 Snails freely make use of mucous filaments to ascend 

 to and descend from the surface to which they must 

 resort for breathing purposes, and are probably also 

 assisted by their mucous secretion when crawling in 



