EVOLUTION II5 
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 
