394 



GASTEROPODA. 



cord ; all the nerves which supply the skin, 

 the muscular integument, the tentacles, the 

 eye, and the muscles of the mouth arise from 

 the brain, and anatomists have not hitherto 

 detected any other source of nervous supply, 

 although Cuvier suspected two minute bodies, 

 which he found beneatli the oesophagus appa- 

 rently connected with the brain, to be of a 

 ganglionic nature. 



The slow-moving and repent tribes of which 

 we are now speaking have their powers of sense 

 almost entirely limited to the perception of 

 objects in actual contact with their bodies, 

 and instruments adapted to touch and vision 

 are the only organs of sense which the anato- 

 mist has been able to distinguish. The utter 

 want of an internal skeleton or of an external ar- 

 ticulated crust forbids us to expect that any of 

 them are provided with an apparatus specially 

 calculated to appreciate sonorous undulations. 

 Their tongue, coated as it is with horny plates, 

 studded with spines, or absolutely corneous in 

 texture, is obviously rather an instrument of 

 deglutition than an organ of taste. No re- 

 searches have hitherto detected any part of the 

 body which could be looked upon as devoted 

 to smell ; the eye is generally a mere point, 

 rather inferred to be such by analogy than 

 clearly adapted to vision ; and the sense of 

 touch in fact is the only one which anatomical 

 evidence would intimate to be perfectly deve- 

 loped. Yet in spite of these apparent defi- 

 ciencies, observation teaches us that many genera 

 are not utterly deprived of the power of appre- 

 ciating intimations from without connected with 

 the perception of odours ; it has been found 

 by direct experiment that some of them are pe- 

 culiarly sensible of the approach of scented 

 bodies ; thus the snail, although at rest within 

 the shelly covering which forms its habitation, 

 will with great quickness perceive the proximity 

 of scented plants which are agreeable articles of 

 food, and promptly issue from its concealment 

 to devour them. Some anatomists have sup- 

 posed that it is at the entrance of the respiratory 

 cavity that we are to look for the special seat of 

 smell, where, as the air alternately enters and is 

 expelled by the movements of respiration, the 

 odorous particles with which it may be impreg- 

 nated are rendered sensible. Others with 

 scarcely less probability conceive that the whole 

 surface of the body which is exposed to the at- 

 mosphere may be endowed with a power of 

 smelling, the quantity of nerves which are dis- 

 tributed to the integument, and the moisture 

 with which it is constantly lubricated, seeming 

 to adapt it perfectly to the performance of this 

 function, giving it all the characters of a 

 Schneiderian membrane. It is not impossible 

 that sounds may be perceived in a somewhat 

 analogous manner, although no proof has yet 

 been adduced that any of the Gasteropoda are 

 sensible to impressions of this nature. The 

 sense of touch is exquisitely delicate over the 

 whole surface of the animal, but more especially 

 so in the foot, which is extremely vascular and 

 abundantly supplied with nerves ; yet in spite 

 of this delicacy in the organisation of the skin 

 which makes it so sensible of contact, it appears 



to have been beneficently ordered that animals 

 so helpless and exposed to injury from every 

 quarter, are but little sensible to pain, and that 

 such is the case, M. Ferussac, a diligent ob- 

 server of their economy, bears ample testimony. 

 " I have seen," says he, " the terrestrial gustc- 

 ropods allow their skin to be eaten by others, 

 and in spite of large wounds thus produced, 

 shew no sign of pain." But besides the sen- 

 satiou generally distributed over the skin, we 

 may observe in most instances organs of variable 

 form which seem peculiarly appropriated to 

 touch. These are the tentacles, or horns as 

 they are usually termed, which occupy a va- 

 riable position upon the anterior part of the 

 animal. 



The tentacles vary in number in different 

 genera : thus in Planorbis we find two, in the 

 generality of cases four ; in a few, as some spe- 

 cies of CEolis, six ; and in Polycera even eight of 

 these appendages are met with. The structure 

 of the tentacles is by no means the same in all 

 the individuals belonging to this class. In the 

 aquatic species they are to a greater or less ex- 

 tent retractile, but can in no case be entirely 

 concealed within the body, as is usual in the 

 terrestrial division ; they are therefore not hol- 

 low, but composed of various strata of circular, 

 oblique, and longitudinal muscular fibres, by 

 means of which they are moved in every direc- 

 tion, and applied with facility to the objects 

 submitted to their examination. In all instances 

 they are plentifully supplied with nerves 

 arising immediately from the brain. Their 

 shape is subject to great variation ; they are 

 usually simple processes from the surface of the 

 body more or less elongated, and in some cases 

 even filiform, as in Planorbis. In Mun:.t 

 (Jig. 193) each tentacle is a thick and fleshy 

 stem, near the extremity of which a smaller one 

 is appended. In Tritonia each tentacle is com- 

 posed of five feathery leaflets, and is enclosed 

 in a kind of sheath which surrounds its base. 

 In Don's the two inferior are broad, Hat, and 

 fleshy, while the superior are thick and club- 

 shaped. In Scyllaa they consist of broad 

 fleshy expansions attached by thin pedicles to 

 the anterior part of the body. In Thcllii/s 

 they are placed at the base of the veil which 

 characterises the animal, but in all cases they 

 are solid and incapable of entire retraction. In 

 the terrestrial Gasteropoda, in which from many 

 causes the tentacles are more exposed to injury, 

 a much more complicated structure is needed, 

 by which these important organs are not only 

 moved with facility in different directions, but 

 which allows them to be perfectly withdrawn 

 into the interior of the body, from which posi- 

 tion they may be made to emerge at the will of 

 the animal : the mechanism by which this is 

 effected will be understood by referring to jig. 

 192, representing a dissection of the common 

 snail, and exhibiting the tentacles in different 

 states of protrusion. Each tentacle (c, </,) is here 

 seen to be a hollow tube, the walls of which are 

 composed of circular bands of muscle, and 

 capable of being inverted like the finger of a 

 glove; it is in fact, when not in use, drawn with- 

 in itself by an extremely simple arrangement. 



