202 THE ACKPHALA. $ 187 
Hach eye 1s composed of a ball formed of a fibrous Sclerotica, which is 
situated upon a small eminence, or is sunken in a contractile prolongation of 
the mantle from which projects a cornea, covered by the general skin. Within 
the sclerotica there is a reddish-brown pigment which is continuous in front 
into a brownish or bluish-green Lis which has a circular pupil ; while behind, 
at the base of the eye, it has the appearance of a kind of Tapetum. This. 
is composed of staff-like corpuscles, which produce that beautiful emerald- 
green appearance of the eyes of certain species. 
The Retina surrounds a vitreous body, composed of non-nucleated cells, 
and which receives in front’a very flattened crystalline lens. The optic 
nerves which enter the eye-ball at its posterior part, are, together with those 
of the tentacles, received from those of the mantle, and especially from the 
marginal branches.” 
The following are the modifications which have already been observed 
with the eyes of these animals: 
With the Ascidiae, there are eight eyes at the entrance of the respiratory 
tube, and six of a deep-yellow color at the entrance of the anal tube. They 
are situated in the special fissures around the openings, and in the midst of 
amass of orange-colored pigment. 
With Pholas, Solen, Venus, and Mactra, these organs are very numerous. 
and non-pedunculated, and are situated at the base of the tentacles sur- 
rounding the two orifices of the siphon. With Cardium, the borders of 
the orifices of the short siphons have an extraordinary oumber of protract- 
ile tentacles which can be protruded through the open valves, each of which 
bears an eye of diamond brilliancy. 
With Tellina, the two borders of the mantle have small, reddish-yellow, 
pedunculated eyes, which are quite numerous at the posterior portions, 
With Pinna, the anterior part of the mantle near the adductor muscle 
has, on each side, about forty brownish-yellow eyes situated upon short pe- 
duncles. But with Arca, and Pectunculus, the numerous reddish-brown 
eyes, usually sessile, are scattered irregularly over the borders of the 
mantle. 
Anomia has about twenty brownish-yellow sessile eyes concealed among 
the tentacles, upon each border of the mantle. With Ostrea, the number 
is still larger; for, for more than a third of the length of the mantle, there 
is a very small short-pedunculated yellowish-brown eye between every 
second tentacle. 
But the beautiful emerald-green eyes of the Pectinea are the most re- 
markable. They are pedunculate and situated between the tentacles of the 
marginal fold of the mantle, being very much more numerous upon the 
side of the plane, than upon that of tlie convex valve. 
2 See Garner, loc. cit. fig.3; Krohn, loc. cit. 
fig. 16, and Grube, loc. cit. fig. 2. 
8 Phallusia, Cynthia, and Clavellina, accord- 
ing to Will, loc. cit. No. 623, p. 102. Grant (Out- 
lines, &v., p. 361) has seen, at least with Phallusia, 
these fourteen eyes. 
4 See Will, loc. cit.p. 100. The color of the eyes 
appears reddish blue with Mactra, and of a 
yellowish brown with others, 
5 See Will, loc. cit. The pupil is an elongated 
oval with Pinna, With Pectunculus pilosus, 
the very numerous eyes are partly isolated, and . 
partly grouped in twenties and thirties. 
6 Beside the figures already cited, all of which 
belong to Pecten and Spondylus, see also those 
which Delle Chiaje (Descriz. &c. Tav. LXXV. 
LXXVI.) has given of the eyes of Pecten. In this 
same genus, W7l/ has seen sixteen to twenty-four 
of these organs upon the convex portion of the 
mantle, and thirty-five to forty-five upon the plane 
portion ; and with Spondylus gaederopus, si 
upon the convex, and ninety upon the plane side. 
