SIGHT IN MOLLUSCA. 477 



eyeball. In the Surinam sprat, the eyes are situated quite on the upper part 

 of the head, so that they are often somewhat above the surface of the water ; 

 the pupil is partially divided, and the lens is also composed of two portions, 

 so that it is supposed that one part of this curious eye is adapted for aerial, 

 and the other for aquatic, vision. There have recently been described by 

 Leuckart, minute lens-like bodies, found in the colored spots upon the sides of 

 the head and body of small fishes of the general Stomias and Chauliodus ; from 

 the presence of these bodies, he regards the spots in question, as examples of 

 true ocelli, or ocular spots ; but this inference may be incorrect. 



Amongst the Mollusca, the Cephalopods are remarkable for the great devel- 

 opment of the eyes, which are larger than in any other non-vertebrate animals, 

 and bear a closer resemblance to the organ of vision in the Vertebrata. The 

 large spherical eyeballs, placed symmetrically on the sides of the head, are in- 

 vested by a wide capsule formed by the integument, which is, at one part, 

 transparent, and supplies the place of the cornea. This covering, however, 

 is not completely closed, but presents an opening through which the sea-water 

 enters, and fills up the space between the eyeball proper and the capsule, so 

 that there is no aqueous humor. In Loligopsis and others, this opening is of 

 very large size, and the anterior portion of the lens projects freely into the 

 sea-water. The lens lies deeply embedded in the vitreous humor, and is di- 

 vided into two unequal parts, one in front of thfe other, which are separated 

 by a delicate interposed membrane, extending from the choroid. The ante- 

 rior part is the smaller one. Each consists of a portion of a sphere, and their 

 substance, though soft externally, is dense within, and contains a firm nu- 

 cleus. The sphericity and density of the lens, suggest a resemblance to the 

 lens of fishes, and are dependent on the same necessity of providing a refrac- 

 tive body of greater power than is needed in animals living in air. The double 

 lens of the eye of a cuttle-fish is, indeed, an extreme example of such a pro- 

 vision. The choroid is of a dark color, and there is a well-developed iris, with 

 a kidney-shaped pupil. By means of the ciliary processes, which are well 

 developed, it is possible that the eye is accommodated to vision at different dis- 

 tances. 



Amongst Pteropoda, the Clio has two eyes, which are placed behind the 

 head, and somewhat resemble a bent cylinder. The Pulmo- and Branchio- 

 gasteropods have, with but few exceptions, black points, or eyes, in the neigh- 

 borhood of the oasophageal ganglion, situated, either at the base of the ten- 

 tacles, as in Limnseus, in the middle of those parts, as in Halyotis, or at the 

 apex, as in the genus Helix or Snail. These simple eyes are dark-colored ele- 

 vations, covered by the soft skin of the tentacle, which constitutes a firm 

 transparent cornea ; behind this, is a globular lens, a cup-shaped choroid 

 membrane, sometimes ending in an iris-like ring, and an optic nerve, expand- 

 ing into retinal elements. In the order of the Heteropods, the eyes are very 

 largely developed. Of the Lamellibranchiata, which are acephalous^ most 

 are destitute of eyes. In some, however, rudimentary eyes are said to exist ; 

 these so-called eye-spots are situated, in the Pecten and others, on the margin 

 of the mantle, between the tentacular appendages, and present the appearance 

 of little shining pear-like bodies, supported on short movable pedicles, and 

 having a pigment layer provided with a silvery stratum ; sometimes they are 

 arranged singly, at other times, in groups of 20 or 30, close to each other. In 

 the Tunicated Molluscoida, pigment spots are found on the central nervous 

 ganglion, but no lens has been discovered in them. 



From the preceding account, it appears that in the Molluscous Class, the 

 eye is gradually simplified, in passing from the complex organ of the Cephalo- 

 pods, through the simpler eyes of the Pteropods, Heteropods, and Gastero- 

 pods, to the still more simple eye-spots of certain of the Lamellibranchiata ; 

 but, in all these cases, a special refractive body or lens is met with, so that 

 perhaps more or less perfect images, at least of near objects, are formed, or, 

 as especially occurs in the more simple forms, a mere concentration of lumin- 

 ous rays takes place, at the back of the eye. In the pigment spots of the 

 Molluscoida, however, so far as is known, no lens exists, but merely pigment- 

 ary and nervous substance in close conjunction ; hence in these animals, sight 



