Conjiection and Locomotion, .5 SI 



9 oz. ; yet the snail creeps under this load at apparent ease. 

 Those which, like the Helices and Trochi, have conical 

 shells flattened at the base carry them upright; but wJien 

 the shell is fusiform, or turreted, it is trailed in nearly a 

 horizontal position, with the point always directed back- 

 wards. The Cypr<^V, 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 Mollusca cover their shells more or less com- 

 pletely with similar expansions. But the Pleurotoma is the 

 most singular of all in this respect. According to Argen- 

 ville, when this Mollusca 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 they 

 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 Aphysia 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 Turbines, 

 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 (Mollusca pulmonifera) which exhibit 

 131 ^ this not unremarkable mode 



of progression in the most 

 perfect manner. On a sum- 

 mer's day any one may see 

 the Lymnae'se {Jig, 131.) and 

 Planorbes thus traversing 

 the surface of ponds and ditches in an easy undulating line f , 

 or suspended there in luxurious repose, perhaps — 



* Lamarck, Anim. sans Vert., vii. 90. If a molluscous animal be touched 

 during progression, it immediately, as it is well known, shrinks and stops for 

 a time : but a terrestrial species (( Helicolimax Lamarckii) 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. 



f MuUer says that in this position no motion of the foot is perceptible. 

 " In fluviatilibus nulla quidem undulatio percipitur ; ope tamen occultae 

 rotationis vel ignoto mechanismo nee lentius, quam terrestres, progrediun- 

 tur." (Hist. Verm. ii. pref. xx.) " In freshwater snails there is no percep- 



