Eyos of Molluscs and Arthropods. 547 



one readily sees, with an ordinaiy band lens, innumerable, small pig- 

 ment spots, which, lipon closer iuspection, prove to be bigbly organized 

 eyes. But before we describe more minutely tbese iuteresting- organs, it 

 will be well to give a general idea of that portion of the mantleupon 

 which they are located. 



In all the species examined, the same di vision of the mantle edge 

 into Shell , Ophthalmie, and velar fold was found (PI. 30, figs. 55 

 and 56), all three extending along the entire edge of the mantle. In 

 seetions, one sees that the shell, or outer fold i, whose inner wall is 

 slightly thickened, is thin and deep [s. h.f.) . The Ophthalmie fold [o.f.) 

 is somewhat thicker and taller than the former ; its free end is thickened 

 and bears the eyes. It is usually covered with dense, dark brown or 

 black pigment, most abimdant on the branchial side. The velar fold is 

 thelargest, {«.) although very insignificant when compared with that of 

 Pecten, and is covered with dark brown pigment, specially abundant 

 at the anterior, and posterior portion of the mantle. 



The velum of Area barbata is well developed and forms a thin, 

 narrow and colorless band in the middle of the mantle, while towards 

 the anterior end it becomes deeply pigmented on both sides, and more 

 than twice as broad as before. 



In all three species, the anterior portion is especially thickened 

 and completely covered with a coat of dark brown, or black pigment. 

 the intensity of which seems to depend upon the health of the individuai 

 animai, and always diminishes with prolonged confinement. If the edge 

 of the mantle be now examined with a pocket lens, a nearly regulär row 

 of dark brown spots will be seen, arranged along the summit of the 

 Ophthalmie fold, larger near the pigmented anterior and posterior thicken- 

 ings , but smaller aud much more numerous in the median portions. 

 None are to be found near the hinge, on the last centimetre of the ante- 

 rior and posterior edge of the mantle. These eyes may be divided into 

 three kinds, the faceted, the invaginate, and the pseudo-lenti- 

 culate eyes, the first named being the most highly developed. They 

 form slightly flattened. hemispherical elevations of the epidermis, cou- 

 fined to the anterior and posterior thickenings of the mantle. 



The surface of the elevations appears to consist of a varyiug 

 number (10 — 80j of perfectly transparent and refractive lenticular pro- 

 jectious of the cuticula, which, when viewed from above, appear like 



1 In speaking of the mantle, the inner surface means towards the branchial 

 cavity, and the outer toward the adjacent shell. 



37* 



