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 Mollisca, 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 
Glatcus Forster7, 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 
sentenced’ and by undulatory motions propagated along its 
surface, resembling, to use the apt comparison of Swammer- 
dam, ‘* the waves “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.t 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 ; 
5 
another of Strémbus gigas is nearly 1 ft. in length. The 
weight of the former is 4 lbs. 20z., that of the latter 4 lbs. 
* Account of the Arctic Regions, vol. 1. p. 544. 
+ 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, Ist, ‘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 cir cles along the whole of its 
length ; and thus, by assuming a ver tical 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, 
ii. 599. 
