GASTEROPOD CRAWLERS. 131 



pedicle which intervenes between the insertion of the foot 

 and the body. In walking, the aperture of the shell is placed 

 plumb on the back, while the spire is laid horizontally, di- 

 rected obliquely to the right, and raised just high enough to 

 avoid contact with the body. This position of the shell is 

 sufficiently singular, but the action of the foot is more so, 

 for at each effort in progression the tail is raised a little, and 

 then beat against the plane of motion to give a greater im- 

 pulse to the foot, or, as it were, a push to the body, while 

 two wide undulations only pass rapidly from the tail forwards 

 to the head. * A structure the very reverse of that of the 

 Pedipes is met with in the Phasianella, so named because of 

 the beautiful pheasant-like disposition of the colours on the 

 shell. MM. Quoy and Gaimard, who had many opportu- 

 nities of studying the large species peculiar to the coasts of 

 New Holland, tell us that when these Gasteropods creep, their 

 foot appears to be divided by the mesial line into two lateral 

 halves which advance alternately : when the right side moves 

 the left remains stationary, and when this is carried forward, 

 the other half of the foot serves as a point of support. 

 Audouin and Milne Edwards have discovered that the little 

 species of the same family found on the shores of some parts 

 of France, exhibit the same peculiarities in their mode of 

 progression, which, they say, may be compared, to a certain 

 extent, to the amble or canter of the horse ;f and you may 

 hope that, in some future day, you shall verify the observa- 

 tion, for we have a Phasianella indigenous on our southern 

 coasts. 



The Gasteropods which are sea-born, and which live more 

 on the ocean than on the coasts, have in general certain aux- 

 iliary appendages to the foot, sometimes so highly deve- 

 loped as to become indeed the principal means of their 

 movements. The Aplysiae, the Aceres, and the bulk of the 

 Mollusca nudibranchia, require the aid of such appendages, 

 for they are liable to be carried away from their littoral 

 haunts by storms, or on floating sea-weed, into deeper water, 

 where they must avail themselves of the fin-like expansions 

 of their cloaks as adjuvants to the power of their foot. Some 

 of them appear to have been created expressly to dwell amid 

 the fields of the floating gulf -weed ; for the foot has been 

 lengthened, and narrowed, and channelled down its middle, 

 so that it may receive the slender frond of the weeds in the 

 furrow, and give a firmer grasp and security to the creature. 

 Of this beautiful adaptation the Scyllaea affords a good 



* Michaud's Supp. to Drap. p. 60. t Litt. de la France, i. 135. 



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