LINNEAlSr SOCIETY OF LONDON. 21 



over whicli it may be making its way. It occasionally (especially 

 when ouly liaJf-grown) indulges in another mode of locomotion. 

 It ceases to glide and to hold on by its ventral surface, and 

 then there is a momentary flexure of one half of the foot — 

 we will say the right half of the foot is suddenly bent bodily 

 towards the middle line, carrying with it to a certain extent 

 the body above, bent in a ventral direction. This sudden 

 bending towards the middle line, the result of the stroke 

 which as it were it gives the water, will cause the animal 

 to heel over to the opposite side. This will be alternated : 

 first the right and then the left half of the foot will thus 

 bend towards the middle line, so that with a sort of rocking, 

 lieaving motion this gasteropod will gradually work its way 

 up the water, and will maintain itself, according to the 

 rapidity of its strokes, at a certain level. Not only does it move 

 in this way, raising itself in the water, and being carried, perhaps, 

 by the currents to a distance, but it actually sometimes moves 

 through the w;iter in a horizontal way, because at the same time 

 that these contractions of the right and left halves of the foot 

 are going on the margin of the foot is undergoing a wave- 

 like contraction from before backwards, and these rij)ple-like, 

 wave-like contractions from before backwards act as a forward 

 propeller of the body, just in the same way as we see a Sole, 

 or one of the great E,ays, swimming by a rij^ple-like undula- 

 tion passing from the anterior border of its tins towards the 

 posterior border; so that in like manner the mollusc will glide 

 forwards by the ripple-like motion of its foot-margin, and at the 

 same time it will raise itself by the flapping movements, as it 

 were, now of this, now of that side of its foot. This, then, is an 

 exceptional way for an ordinary gasteropod to move. 



There is another gasteropod which I will mention that has 

 an extraordinary way of moving, fitting it, however, most ad- 

 mirably to the conditions of its existence. It is unfortunately 

 one we shall never see alive at the Marine Biological Station at 

 Plymouth, for, being a tropical form, it is not found alive on 

 our coasts. I only mention it because there are so many 

 features in its structure which are of interest. The species in 

 question inhabits a shell in the form of a very flattened cone. 

 From the mouth of this shell there protrudes that which increases 

 the size of the shell by secretion from its outer surface, 

 namely, a sort of fleshy mantle-lobe which extends beyond the 

 margin of the mouth, and then from the mouth of the shell, when 

 the thing is actively moving, there passes down a cylindrical 

 body ; and in front of this cylindrical body we see the head- 

 region with an everted proboscis and a couple of filamentous 

 tentacles, with a bright little eye at the base of each; and below, 

 ventrally, we have the foot, consisting of two divisions — the 

 propodium or anterior division, and the Qnesopodium or middle 

 division. Then there is a third, or posterior, division of the foot — 



