26*2 ORGANS OP THE SENSES. 



pupilar opening of the choroid behind the transparent skin 

 which formed a distinct convex corneee over the organ, and 

 two small muscles behind for the motion of this sessile eye. 

 A similar structure nearly was found by Muller in the large 

 eyes of the murex tritonis (Fig. 103. C. D. E), where they 

 occupy their most frequent position at the outer part of the 

 base of the tentacula (103. C. a). In a few of the gastero- 

 pods they are placed on the inner part of the base of 

 the tentacula. The delicate skin of the tentaculum (C. a), 

 forms a thin transparent cornea (C, b), over the eye and 

 leaves a considerable chamber for the thin aqueous fluid 

 anterior to the iris (103. D. b,) and the pupil (103. D. a). 

 The eye is lengthened in the direction of its visual axis, and 

 the black circular iris (103. C. d. D. b), continued from the 

 coloured choroid, presents a large round pupil directed 

 obliquely outwards from the base of the tentaculum. The 

 interior of the large cavity surrounded by the choroid and 

 its pigment, appears to be chiefly occupied by the irregular 

 round pellucid amber-coloured mass of the crystalline lens 

 (103. E), and the optic nerve (103. C. e), which comes off 

 from the common tentacular nerve, enters the eye-ball ob- 

 liquely on the outside of the axis of vision. The organs of 

 vision, two in number and placed on the sides of the head in 

 the pteropods, though small in their dimensions, have pro- 

 bably an internal structure similar to that found in most of 

 the gasteropods. The position of these organs is seen in the 

 cymbulia of Peron (Fig. 24. c) and in the clio of the South 

 Seas (Fig. 6J. A, b), they are also obvious on the head of the 

 cleodora. In the cephalopodous animals these organs pre- 

 sent the greatest size and the most complicated structure met 

 with in any of the invertebrated classes, and they are even of 

 great size in proportion to the magnitude of the head and to 

 the general bulk of the body. As in the higher Crustacea, 

 and in some of the gasteropods and cartilaginous fishes, the 

 eyes are sometimes pedunculated in the animals of this class 

 as in the nautilus and the loligopsis (Fig. 93. A. c), and they 

 already present distinct external muscles for their movements, 

 and palpebral folds of the surrounding skin, which passes 

 transparent like a conjunctiva over their anterior surface to 

 form a smooth cornea, as in other mollusca. Like the eyes 

 of fishes, they are generally flattened in front from the defici- 

 ency of aqueous humour, and they are of great size from the 



