GASTEROPODA. 



403 



servient to mastication, the apparatus of the 

 external senses, and of the organs employed in 

 copulation, which are, when unemployed, re- 

 tracted within its cavity by special muscular 

 fasciculi spoken of elsewhere. 



Retractile muscles. In the spirivalve genera 

 the muscular walls which inclose the body only 

 exist in such parts as, during the extended state 

 of the animal, are protruded from the shell ; 

 that part of the body which is concealed within 

 its cavity being provided with a much more 

 delicate and membranous envelope ; in such, 

 however, a necessity exists for an additional 

 muscular apparatus, serving to retract the body 

 and foot within the cavity of its calcareous 

 abode, and of course exhibiting various modifi- 

 cations of arrangement in conformity with the 

 shape of the shell itself. In the turbinated 

 shells, the retracting muscles consist of strong 

 fasciculi of fibres arising from the columella 

 or axis of the shell, and diverging from this 

 point, spread in several slips, which become 

 interlaced with the fibres composing the foot 

 and muscular investment. In the flattened 

 forms of Patella and Chiton, the muscular 

 fibres arise all around the margin of- the shell, 

 excepting at its anterior part; these penetrating 

 the mantle are intimately interwoven with the 

 muscles forming the circumference of the foot. 

 The animal of the Haliotis is fixed to its ex- 

 panded and semi-turbinated shell by a single 

 large and ovoid muscle, which takes its origin 

 from near the middle of the last spire ; what- 

 ever the disposition of these muscles, however, 

 their action is obviously of two kinds; and not 

 only are they the agents by which the creature 

 retires within its covering, but by raising the 

 central portion of the disc of the foot, whilst its 

 margins are in apposition with the plane of 

 progression, they will, by producing a vacuum 

 beneath, convert the whole apparatus into a 

 sucker, the adhesive power of which will be 

 proportioned to the extent of its surface. 



Foot. The foot of the Gasteropoda is their 

 principal agent of progression. It is generally 

 a fleshy disc, of variable size and shape, attached 

 to the ventral surface, and forming when ex- 

 panded an organ by means of which the animal 

 can adhere to surrounding objects. In the 

 naked genera it is small, but in the conchife- 

 rous species, especially in such as are provided 

 with dense and weighty shells, its dimensions 

 and force are proportionally increased. In its 

 internal structure it resembles the muscular in- 

 vestment of the body, of which in fact it is 

 merely an expansion, consisting of muscular 

 fibres interlacing each other in every possible 

 direction, as may be developed by continued 

 maceration. In the Slug, when opened from 

 the back, the superior layer of fibres is found to 

 run transversely, arising apparently from two 

 tendinous lines which run longitudinally near 

 the centre of the organ, and terminating near 

 the margins of the disc ; beneath these, longi- 

 tudinal fasciculi may be detected, but so inter- 

 laced with other fibres assuming every degree 

 of obliquity, that it is impossible to unravel the 

 complicated structure which they form. In the 

 Limpet (Patella) the lower fibres of the foot 



are transverse, but near the circumference they 

 become distinctly interwoven with circular fas- 

 ciculi ; the superior stratum viewed from above 

 consists of two series of oblique fibres, which 

 meet at an acute angle on the middle line, 

 whilst the substance of the organ is composed 

 of muscular bands variously disposed : from 

 such a structure the movements of the foot are 

 readily understood ; the transverse fibres by 

 their contraction will elongate the ellipsis of 

 the foot by diminishing its breadth, whilst the 

 longitudinal, having a contrary action, will, by 

 the combination of their effects, produce every 

 movement needful for the progression of the 

 creature. On minutely inspecting the foot of 

 a terrestrial Gasteropod, as it crawls upon a 

 transparent surface, it will be found to be 

 divided into a certain number of transverse seg- 

 ments of variable size by a particular arrange- 

 ment of the longitudinal muscular fibres, which 

 seem to form, when the creature advances, un- 

 dulations limited by the points of contact. 

 These sections appear alternately to form a 

 vacuum upon the surface where the animal is 

 placed, that which follows advancing to take 

 the place of that which precedes it, the trans- 

 mission of movement occurring from behind 

 forwards, a mechanism which causes the animal 

 to advance by a slow and uniform progression. 

 The above structure of the foot, and conse- 

 quent mode of locomotion, although the most 

 usual, is susceptible of considerable modifica- 

 tion. Thus in Scyllaa, we find it only adapted 

 for grasping the thin steins of fuci and other 

 submarine plants, being for that purpose com- 

 pressed and grooved inferiorly into a deep sul- 

 cus. In the Tornatellafasciata, Lam. the struc- 

 ture of the foot is remarkable : beaten incessantly 

 by the waves, in the cavities of rocks which it 

 frequents, nearly on a level with the surface of 

 the sea, to the violence of which it is always 

 exposed, it has need of additional powers of 

 retaining its hold; its foot is therefore divided 

 into two adhering portions, placed at each 

 extremity, and separated by a wide interval ; 

 when it crawls it fixes the posterior disc and 

 advances the other, which it attaches firmly to 

 the plane of progression, and this being effected, 

 the hinder sucker is detached and drawn for- 

 wards, locomotion being accomplished by the 

 alternate adhesion of these two prehensile discs. 

 In Cyclostoma the foot is likewise furnished 

 with two longitudinal adhering lobes, which 

 are advanced alternately. But the foot is not 

 merely an instrument of progression on a solid 

 surface, in many species being convertible, at 

 the will of the animal, into a boat, by means of 

 which the creature can suspend itself in an in- 

 verted position at the surface of the water, 

 where by the aid of its mantle and tentacles, it 

 can row itself from place to place. The Buli- 

 mus stagnalis, so common in our pools of fresh 

 water, is a good example of this mode of sail- 

 ing ; and in the marine species, Aplysia and 

 Gastropteron may be enumerated as exhibiting 

 a similar structure. 



Some of the naked Gasteropods, as Aplysia 

 and Thethys, are able to move through the 

 water in the same manner as the leech by an 



