ORGANS OF SENSE IN MOLLUSKS. G19 



to the Mouth, so as to supply the Digestive apparatus with the 

 aliment afforded by the Diatomacece, Infusoria, &c, which it 

 carries-in with it. 



482. Organs of Sense of Mollusks. — Some of the minuter and 

 more rudimentary forms of the special organs of Sight, Hearing, and 

 Touch, which the Molluscous series presents, are very interesting 

 objects of Microscopic examination. Thus just within the margin 

 of each valve of Pecten, we see (when we observe the animal in its 

 living state, under water) a row of minute circular points of great 

 brilliancy, each surrounded by a dark ring ; these are the Eyes, 

 with which this creature is provided, for the purpose (it can scarcely 

 be doubted) of directing its peculiarly-active movements. Each of 

 them, when their structure is carefully examined, is found to be 

 protected by a Sclerotic coat with a transparent Cornea in front, 

 and to possess a coloured Iris (having a pupil) that is continuous 

 with a layer of pigment lining the sclerotic, a Crystalline lens and 

 Vitreous body, and a Retinal expansion proceeding from an Optic 

 nerve which passes to each eye from the trunk that runs along the 

 margin of the mantle. Eyes of still higher organization are borne 

 upon the head of most Grasteropod Mollusks, generally at the base 

 of one of the pairs of tentacles, but sometimes, as in the Snail and 

 Slug, at the points of these organs. In the latter case, the ten- 

 tacles are furnished with a very peculiar provision for the protec- 

 tion of the eyes ; for when the extremity of either of them is 

 touched, it is drawn -back into the basal part of the organ, much 

 as the finger of a glove may be pushed-back into the palm. The 

 retraction of the tentacle is accomplished by a long muscular slip, 

 which arises within the head, and proceeds to the extremity of the 

 tentacle ; whilst its protrusion is effected by the agency of the 

 circular bands with which the tubular wall of the tentacle is itself 

 furnished, the inverted portion being (as it were) squeezed-out by 

 the contraction of the lower part into which it has been drawn back. 

 The structure of the Eyes, and the curious provision just described, 

 may easily be examined by snipping-off one of the Eye-bearing 

 tentacles with a pair of scissors. — None but the Cephalopod Mollusks 

 have distinct organs of Hearing ; but rudiments of such organs may 

 be found in most Gasteropods (Fig. 323, k, x), attached to some 

 part of the nervous collar that surrounds the (Esophagus ; and even 

 in many Bivalves, in connection with the nervous ganglion imbedded 

 in the base of the Foot. These ' Auditory Vesicles, ' as they are 

 termed, are minute sacculi, each of which contains a fluid, wherein 

 are suspended a number of minute Calcareous particles (named 

 Otolithes or ear-stones), which are kept in a state of continual 

 movement by the action of Cilia lining the vesicles. This "won- 

 derful spectacle," as it was truly designated by its discoverer 

 Siebold, may be brought into view without any dissection, by sub- 

 mitting the head of any small and not very thick-skinned Gras- 

 teropod, or the young of the larger forms, to gentle compression 



