594 William Patten 



matoxylin, the argentea never, to my knowledge, absorbs coloriug 

 matter. Krohn first described the argentea of Pecten^ and ascribed to 

 it the luminoiis appearance of these eyes. Will appeared to confoimd 

 it with the tapetum. Hensen seems to have seen the minute , Square 

 plates of the membraues, for he mentions the »Stäbchenförmige Mole- 

 cule« , whieh he is iuclined to regard as eells. Hickson saw the play 

 of color produced by the argentea, which he considered gave the eyes 

 their »beautiful metallie lustre«. The argentea is formed according to 

 him »of a great number of fine fibrils crossing at right au- 

 gi es«. Carrière states that. »Das Tapetum lucidum besteht aus feinen 

 Fasern , welche alle in derselben Richtung verlaufen, und zwar, gleich 

 den Seitennerven innerhalb des Auges , quer (senkrecht) zu der Ober- 

 fläche des Mantels«. 



The tapetum, the red pigment layer of previous writers, usually 

 consists of a single layer of cells, decreasing in thickness from the axial 

 part of the eye toward the periphery, and terminating, with the argentea, 

 at the entrance of the fibres from the axial brauch of the optic nerve 

 into the retina. The cells are large, many-sided bodies filled with 

 coarse, red pigment granules, in the midst of which is a round, or oval, 

 vesicle-like nucleus containing, in a clear fluid, two or three deeply 

 stained and irregularly shaped granules , or nucleoli. The red color is 

 preserved excellently well in hot sublimate, while it is dissolved or 

 destroyed in alcohol, or in combinatious of picric or chromic acid. 



The tapetum forms one of the most conspicuous objects in the eye, 

 and, for that reason, has long been known. It was first described by 

 Keohn, but Hensen was the first to discover its cellular nature. Hick- 

 son was subsequently unable to confirm Hensen's observations, andcame 

 to the conclusion that »the pigment contains no cellular ele- 

 ments at all«! Carrière has correctly described and figured the tape- 

 tum, which he erroneously considers to be a continuation of the nu- 

 cleated septum. It must be considered, however, as continuous with 

 the retina, and homologous with the outer ganglionic cells, as we shall 

 explain more fully in treatiug the development of the eye; (see diagram, 

 PI. 32, figs. 151 — 152). Acircumstance, which Icannot regard as acciden- 

 tal, is that, in the eyes of all the species oiPecten examined, the tapetum 

 is not surrounded by pigment, but is exposed to the light from below. 

 The iris always extends as far inwards as the outer edge of the tapetum, 

 and there abruptly terminates. On the shell side, the iris, as has already 

 been described, extends much further towards the mantle. and forms a 

 narrow, pigmented band ou that side of the stalk. AVe have failed to 



