306 LECTURE XXII. 



vesicles. The functions of tlie other ganglions of the body seem 

 to be limited to the automatic reception and reflection of stimuli. 



Soft and lubricated and sensitive as the skin of the naked MoUusks 

 seems to be, there are not wanting reasons for supposing it to be 

 possessed of a very low degree of true sensibility. Baron Ferussac, 

 for example, states that he has seen the terrestrial Gasteropods, or 

 slugs, allow their skins to be eaten by others, and, in spite of large 

 wounds thus produced, show no sign of pain. 



The vascular inferior surface of the foot can, doubtless, take cog- 

 nisance of the character of the surface over which it glides ; but the 

 special organs of the tactile sense are the tentacula or horns which 

 project from the lateral and upper parts of the head. These are 

 wanting in a few Gasteropods, hence called Akera : they are some- 

 times two, and never exceed four, in number in the present class. In 

 the snails and slugs they can be retracted by an act of inversion. 

 The anterior is the normal or constant pair; the posterior pair, 

 which supports the eyes in the snail, is reduced to two short pro- 

 cesses, which extend from behind the basis of the anterior tentacula 

 in the Turbo; and which form slight projections from the outer side 

 of the base of those tentacula themselves in the Paludina (^Jig. \22.), 

 and in most Pectinibranchiata. In the Aplysia, however, which has 

 four tentacula, the ocelli are sessile, and situated in advance of the 

 bases of the posterior pair. 



The eyes never exceed two in number in the Encephalous Mol- 

 lusca : in the Gasteropods they present their largest relative size in 

 the Pectinibranchiata. In this preparation * from a large species of 

 Murex, you may readily discern the sclerotic tunic with its anterior 

 orifice, the expansion of the optic nerve posteriorly between the fibrous 

 and the pigmental tunic, and the large spherical crystalline lens, 

 covered anteriorly by the transparent corneal integument ; between 

 which and the lens, there is a very small interspace for the aqueous 

 humour and the pupillary circular opening left by the pigmental 

 layer or choroid. 



The existence of the sense of hearing in the Gasterojiods was 

 inferred by Dr. Grant, long before the organ was detected: he justly 

 concluded that the sounds emitted by the Tritonia arhorescens under 

 water, were doubtless intended to be heard by others of the same 

 species. The very general existence of an acoustic appai'atus under 

 its most simple conditions, in the lower Mollusca, has been esta- 

 blished by the discoveries of Siebold. It consists of two round 

 vesicles, containing fluid and crystalline or elliptical calcareous par- 

 ticles, or otolithes, remarkable for their oscillatory action in the living 



* No. 1628. 



