» Connection and Locomotion. 531 
9 oz.; yet the snail creeps under this load at apparent ease. 
Those which, like the Hélices and Zréchi, have conical 
shells flattened at the base carry them upright; but when 
the shell is fusiform, or turreted, it is trailed in nearly a 
horizontal position, with the point always directed back- 
wards. The Cypre’@, when they walk, cover their shell 
with the lateral lobes of their cloak, which are very often 
beautifully and vividly marked with various colours; and 
many other Mollisca cover their shells more or less com- 
pletely with similar expansions. But the Pleurdtoma is the 
most singular of all in this respect. According to Argen- 
ville, when this Mollisca creeps, it elevates and sustains its 
shell and cloak upon a rather long peduncle or stalk, which 
rises vertically from the back. In consequence of this remark- 
able position of the shell, the animal tumbles over at every 
impediment; but it heeds not, quietly resumes its proper 
attitude, and pursues the road.* 
All Gasteropodes are not confined, however, to crawl on 
the solid bottom: many of them can ascend to the surface, 
and make the waters a liquid pavement, along which the 
creep, in the same manner as they do on land, with the differ- 
ence only of having their body and shell in a reversed position. 
I have observed the once terrible Aph¥sia depilans crossing 
pools on our shore in this way; and there is some reason to 
believe that all the marine naked Mollusca possess this faculty. 
When I have confined a number of the minute Tiarbines, 
sO common on our coasts, in a glass of sea water, some have 
very soon suspended themselves from the surface; but it is 
the freshwater snails (Mollasca pulmonifera) which exhibit 
131 this not unremarkable mode 
a ie of progression in the most 
CGC =XV perfect manner. On a sum- 
mer’s day any one may see 
the Lymnee‘ee (jig. 131.) and 
Planorbes thus traversing 
the surface of ponds and ditches in an easy undulating line +, 
or suspended there in luxurious repose, perhaps — 
))) a 
* Lamarck, Anim. sans Vert., vii. 90. Ifa molluscous animal be touched 
during progression, it immediately, as it is well known, shrinks and stops for 
a time: but a terrestrial species (Helicolimax Lamarcki) affords an excep- 
tion ; for, “if disturbed or irritated, it only crawls the faster; and, if at rest 
and contracted, it directly puts itself in motion on being touched or dis- 
turbed.” —- Lowe in Zool. Journal, iv, 342. 
+ Muller says that in this position no motion of the foot is perceptible. 
“ Tn fluviatilibus nulla quidem undulatio percipitur ; ope tamen occulta 
rotationis vel ignoto mechanismo nec lentius, quam terrestres, progrediun- 
tur.’ (Hist. Verm, ii. pref. xx.) “ In freshwater snails there is no percep- 
