202 THE ACEPHALA. § 187. 



Each eye is 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 Iris which has a circular pupil ; while behind, 

 at the base of the eye, it has the appearance of a kind of Tapetum. This 

 is compos'ed 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 tfeose 

 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 n)idst of 

 a mass of ^orange-colored pigment.'^* 



With Pholas, Solen, Venus, and Mactra, these organs are very numerous 

 and non-pedunSulated, 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 extraordinarj' number 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 Piiina, 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 Area, and Pectunculus, the numerous reddish-brown 

 eyes, usually sessile, are scattered irregularly over the borders of the 

 mantle.*'' 



Anotnia 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 the convex valve. <®> 



2 See Garner, loc. cit. fig. 3; Krohn, loc cit. the very numerous eyes are partly isolated, and 

 fig. 10, and Grube, loc. cit. fig. 2. partly grouped in twenties and thirties. 



3 Phallusia, Cynthia, and Clavellina, accord- ^ Beside the figures already cited, all of whicb 

 ing to Will, loc. cit. No. 623, p. 1(V2. Grant (Out- belong to Pe.cten and Spondylus, see also those- 

 lines, &c., p. 361) has seen, at least with PAaWusia, wiiich Delle Chiaje (Descriz. &c. Tav. LXXV. 

 tliese fourteen eyes. LXXVl.)has given of the eyes of Pecten. In this 



i See IVill, loc. cit. p. 100. The color of the eyes same genus, JVill has seen sixteen to twenty-four 



appears reddish blue with Mactra, and of a of these organs upon the convex portion of the- 



yellowish brown with others. mantle, and thirty-five to forty-five upon the plaiiu 



5 See fVill, loc. cit. Tlie pupil is an elongated portion ; and with Spondi/lus gaederopus, si.\ty 



oval with Pinna. Witli Pectunculus pilosua, upon the convex, and ninety upon the plane side. 



