TENTACLES OF PECTEN. 527 



pared mind, than tlie exhibition of such wonderful activity as 

 will then become apparent, in a body which to all ordinary ob- 

 servation is so inert. This activity serves a double purpose ; for 

 it not only drives a continual current of water over the sur- 

 face of the gills themselves, so as to effect the aeration of the 

 blood, but it also directs a portion of this current (as in the Tu- 

 nieata, § 333) to the mouth, so as to supply the digestive appara- 

 tus with the aliment afforded by the Diatomacece, Infusoria, kc, 

 which it carries in with it. 



354. Organs of Sense of Mollushs. — Some of the minuter and 

 more rudimentary forms of the special organs of sight, hearing, 

 and touch, which the Molluscous series presents, are very inte- 

 resting 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 struc- 

 ture is carefully examined, is found to be protected by a sclerotic 

 coat with a transparent cornea in front, and to possess a colored 

 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 Gasteropod 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 curi- 

 ous 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, attached to some part of the nervous collar that 

 surrounds the oesophagus ; and even in many Bivalves, in con- 

 nection 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 



