182 SENSES — TOUCH. 



neus ; in some Aplysiee tliey are blackish-red and granular ; 

 and in our native Aplysiae they are yellow. Carus asserts, 

 that those of the common fresh-water mussel are invariably 

 bright yellow;* in the Arcae, Pinnae, and a few others, they 

 are tinted a rose-red, but in the majority of Bivalves they are 

 yellowish, or almost white, and soft and transparent. 



I will now tell you all I know about the senses of the 

 mollusca, and will begin with that of touch, as it is common 

 to the whole class, and consequently the most important. 

 It is, indeed, doubtful whether the bivalved mollusca have 

 any other sense ; they have no undisputed eyes, no true 

 tentacula, no tongue, slight vestige of an ear, and if they 

 possess the perception of smells, we know no organ in which 

 it is localized; — "neither, indeed," says Bradley, "do I 

 think the necessary organs for those senses can reasonably be 

 sought for in such bodies as have a fixed state of life ; the 

 senses of feeling and tasting being sufficient for the main- 

 tenance and suj)port of them." 



I. TOUCH. 



The skin of the mollusca is a soft, spongy, mucous mem- 

 brane, wrinkled and thickish where exposed, smooth and 

 very thin where covered with the shell. It is never in the 

 slightest degree hairy, or villous, or horny, but always kept 

 in a moist state by a glutinous secretion, exuded in some 

 instances from " little, glandulous, unequal grains," pro- 

 fusely scattered over the surface ; in others, from crypts or 

 glands confined to particular parts. It is a homogeneous 

 membrane, not divisible into epidermis and cutis vera, like 

 the skin of the vertebrate animals ; and it is so intimately 

 fixed to, or rather interwoven with, the subjacent muscular 

 layer, that it is contractile at every point, and in all direc- 

 tions. It invests every part, sometimes closely, but more 

 commonly there is "ample room and verge enough" to 

 form folds and expansions ; from which circumstance it has 

 received the name of mantle or cloak. The blood-vessels 

 distributed in its texture are very numerous, and the nerves 

 are presumed to be at least equally so. 



From this structure we might have concluded that the 

 skin would be pecuharly sensible to external impressions ; 

 and this we know is the fact. Let your experiment be 

 made with the lightest hand and the softest instrument, 

 yet it cannot come into contact with the mollusk which 



* Comp. Anat. trans, i. 53 ; also, Edinb. New Phil. Journ. vii. 229. 



