528 Natural History of Molluscous Animals : — 



from the brain two very long cords are sent out to establish a 

 communication with the ganglion appropriated to the loco- 

 motive organs, and which is found below the posterior adduc- 

 tor muscle.* 



1 will now tell you all I know about the senses of the Mol- 

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

 the whole class, and by far the most important. 



1. Touch, — The skin of the Mollusca is a soft, spongy, 

 mucous membrane, 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," 

 profusely 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 directions. 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 tex- 

 ture 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 peculiarly 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 Molluscum which will not feel the im- 

 pression, and evidence its alarm by intelligible signs. The 

 intimations which they receive by this medium are probably 

 of a very general character, and have respect only to the 

 motion, the temperature, the hardness or softness, of the 

 impressing body. To judge of the position, and perhaps in 

 some degree of the form, of bodies, they are provided with 

 special organs, which, when situated on the head, or about 

 the mouth, are denominated tentdcula ; but, when arranged 

 along the side«^ or on the margins of the cloak, more com- 

 monly tentacular ^filaments. The former are two or four in 

 number, of a cylindrical, tapered, or triangular figure, very 

 flexible, and almost always capable of being withdrawn within 

 a sheath or under the collar, at the will of the animal. These 



* This view, sufficiently extensive and accurate for our purpose, is 

 translated from Blainville's Manuel, p, 142, et seq. 



