GASTEROrOD SWIMMERS. 129 



I have noticed that the Valvata, when in confinement, 

 will remain days shut up in this manner; the Limnaeus stag- 

 nalis is less obstinate, but still a great alarmist ; while the 

 Physae, nearly alhed to the Lymnasus, and apparently less 

 defenceless, are only momentarily arrested by a touch, when 

 they again extend themselves and go on unhesitatingly. But 

 the Helicolimax Lamarckii — a terrestrial species — affords 

 quite an exception to the general rule, for " if disturbed or 

 irritated it only crawls the faster ; and if at rest and con- 

 tracted, it directly puts itself in motion on being touched or 

 disturbed." * The Nanina, an East Indian land mollusk 

 discovered by Mr. Benson, has the same habit as the Helico- 

 limax. f 



All Gasteropods are not confined, however, to crawl on 

 the solid bottom : many of them can ascend to the surface, 

 and make the water a liquid pavement, along which they 

 creep in the same manner as they do on land, with the dif- 

 ference only of having their body and shell in a reversed 

 position. The Aplysiae, and many of our nudibranchial 

 moUusca, may be seen crossing pools on our shore in this 

 way ; and there is reason to believe that all the marine naked 

 mollusca possess the faculty. ;{: When I have confined a 

 number of the minute Rissoffi, 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 

 ^iuhnonifera) which exhibit this not unremarkable mode of 

 progression in the most per- 

 fect manner. On a sum- '»■ :" 

 mer's day any one may see 

 the Linmsei (Fig. 21) and 

 Planorbes thus traversing 

 the surface of ponds and 

 ditches in an easy undulat- 

 ing line ; § or suspended there in luxurious repose, perhaps — 



" To taste the fresliness of lieaven's breath, and feel 

 That light is pleasant, and the sunbeam warm." 



* Lowe in Zool. Journ. iv. 34iJ. 



t Proc. Zool. Soc. 1834, p. 89. 



X Ri.«so, I'Europ. Merid. iv. 39, 47. 



§ Miillcr says that in tliis position no motion of the foot is perceptible. 

 " In flnviatilibus nulla quidem undulatio pcrcipitur ; ope tamen occultse 

 rotationis vol if,nioto meclianismo nee lentius, quam terrestrcs, progrediun- 

 tur." — Hist. Verm. ii. pref. xx. Lister had previously made the same 

 observation. — Excrcit. Anal, de Coch. 153. 



Quatrefages is of opinion tiiat the jirogression of mollusks in this reversed 

 ])osition on the surface of the water, cannot be made by any muscular 



K 



