530 Natural History of Molluscous Animals : — 



" brings the tips of its fins almost into con- 

 tact, first on one side and then on the other." * 

 {fig. 130.) 



Of the Gasteropodous Mollusca, which em- 

 brace all the slug-like species, and nearly all 

 those covered with a univalve shell, a very 

 few only can be said properly to swim. One of them, the 

 Glaucus Forsterf, swims on the surface with a rapidity un- 

 exampled in the class ; and the curious Tethys can swim 

 very well by means of the large semicircular expansion of 

 its cloak, which rises like a tippet above the neck. But 

 these are exceptions, for almost all are doomed to crawl upon 

 the belly, at a pace proverbially slow. The inferior surface 

 of the body is formed into an oval or oblong disk, of a firm 

 texture, composed of muscular fibres, which run, some in a 

 transverse, and some in a longitudinal direction, but so closely 

 interwoven, as not to be separable into distinct layers. This 

 foot, as it is called, is susceptible of being lengthened and 

 shortened ; and by undulatory motions propagated along its 

 surface, resembling, to use the apt comparison of Swammer- 

 dam, " the weaves and billows of the sea," the Gasteropode 

 moves forward in a continuous manner, marking its track, in 

 the land species, with a silver line of concrete slime exuded 

 to smooth the asperities of the road.f You cannot fail to 

 have noticed the snail in its pilgrimage ; and ' the aquatic 

 tribes progress in precisely the same way, whether they slowly 

 traverse the floor of ocean, or climb the rugged steeps of the 

 rock, or stray amongst their groves of sea-weed and coral. 

 To their progress the shell, one would imagine, must prove 

 a serious obstacle, both by its occasional size and weight. A 

 fine specimen of the Cassis tuberosa, in my cabinet, measures 

 fully 10 in. in length, and upwards of 8 in. in breadth ; 

 another of >Str6mbus gigas is nearly 1 ft. in length. The 

 weight of the former is 4 lbs.. % oz., that of the latter 4 lbs. 



* Account of the Arctic Regions, vol. i. p. 544. 



f This is the usual account, but, according to Mr. Main, it is erroneous ; 

 the muscular motions, instead of being from head to tail, being propagated 

 in the contrary direction ; so that the animal's motion cannot be caused by 

 impulses in the direction of its progress. He gives two conjectures as to 

 the cause of the animal's motion ; namely, 1st, that the body is moved for- 

 ward by the retromissive discharge of slime, which, being emitted simul- 

 taneously from every part of the under surface, he conceives, may exercise 

 a force adequate to the propelling of the animal ; or, 2dly, from its power 

 of forming its lower surface into segments of circles along the whole of its 

 length J and thus, by assuming a vertical vermicular action on the plane of 

 the sustaining surface, impelling the body forward by alternate contraction 

 and expansion. As dry air deprives the animal of motion, Mr. Main is 

 inclined to consider the first surmise the more probable. See Zool. Journ.y 

 iii. 599. 



