LOCOMOTION — RATE OF PROGRESSION. 
.849 
In the Helices and other terrestrial genera the locoinotory disc 
presents a greater and more varied proportional area, and the rate 
of progress varies considerably in the different species, the smaller 
forms being relatively or even actually more nimble than the larger 
ones. Helix cantiaim travelling 75 millimetres or more per minute 
upon a horizontal surface or at the rate of a mile in about 14 days 
16 hours, a rate which is not exceeded by even Helix aspersa. 
Cyclostoma elegans, to avoid or overcome the frictional resistance of 
close contact with the soil, while crawling, alternately raises from the 
ground and extends forward each lateral half of its longitudinally 
divided foot, seeking also to aid its progress by the accessory fixation 
of the snout, which is moistened by the secretion of special mucus 
glands, and yet it scarcely progresses 25 millimetres per minute, or 
at the rate of a mile m 44 days. 
The aquatic branchiate species crawl upon the bed of the waters 
they inhabit or over and upon submerged objects, and although the 
greater density of water renders the weight of their shell less burthen- 
some than it would be upon land, their average progress is but about 
25 millimetres per minute or a mile in 44 days. 
The Basommatophora, in addition to peregTinatious upon the bed 
of the stream or pond, can by virtue of their greater buoyancy, due 
to the air within the respiratory cavity, float upon the surface and 
crawl with inverted shell upon the under side of the water-film by 
the successive undulations of the foot-sole as in the terrestrial species, 
these undulations being recorded by the transversely ridged mucous 
track left behind upon the surface of the water (fig. 607, p. 316); 
their progress is, however, aided by the strong vibratile cilia clothing 
the tentacles and the margins of the head and body and is stated to 
average 50 millimetres per minute or a mile in 22 days, although 
Ancyliis Jluviatilis has been recorded to travel only a little more 
than one millimetre per minute, or a mile in 2 years and 10 months. 
Special observation at a temperature of 66° Fahr. showed that a 
moderate sized Limncca peregra could crawl 75 millimetres per 
minute or a mile in 14 days 16 hours, while a half-grown Planorbis 
corneus, under similar conditions, accomplished 88 millimetres in 
the same space of time or a mile in 1 2 days 1 1 hours. 
The Pelecypoda are very sluggish in their movements, the hatchet- 
shaped foot being more especially adapted for ploughing through sand 
and mud, and pi’obably thereby disturbing the minute organisms 
