THE EYE OF PECTEN. 97 



probably by tlie former, since some of the granules con- 

 tained in tliese cells may resemble the substance of the 

 tapetum. 



The pigment layer was also an early discovery because 

 of its conspicuous appearance, and it is often possible to see 

 the red pigment through the substance of the eye-stalk if 

 there is little pigment in the epithelium of the latter. This 

 layer was Patten's tapetum. Hickson had regarded it as a 

 fluid with no cellular elements at all. Carriere thought it 

 was a continuation of the septum, and Rawitz describes it as 

 being differently coloured in the various species. Schreiner 

 explains Hickson's view on the grounds that iu P. maxim us, 

 which he examined, the pigment was really a fluid mass 

 containing large and small granules, but adds that in other 

 species this layer is a single or double row of rather large 

 polygonal cells. 



I have investigated several species and find that this layer 

 is cellular in all, though the boundaries of the cells may be 

 difficult to see in the adult. In young specimens of Pecten, 

 only a few millimetres in diameter, the pigment-layer appears 

 to be composed of a single layer of epithelial-like cells with 

 little or no pigment present. 



As the eye grows the pigment increases, the cells become 

 filled and usually very irregular in shape, so that in large 

 eyes of P. maximus the epithelial ai-rangement persists 

 often only in the marginal part, and in the middle the layer 

 may be irregularly two cells thick. 



The actual colour of the pigment is of little importance, 

 since it varies in specimens of the same species and often in 

 cells of the same eye. It is some shade of red-brown, and 

 generally the cells are filled with a finely granular dark 

 brown pigment, but with here and there frequently large, 

 more darkly coloui'ed bodies, like round concretions (PI. 6, 

 fig. 1, Ta. c). There are large and small bodies of this 

 nature, and sometimes also iridescent granules resembling in 

 appearance the substance of the tapetum. The nuclei are 

 best seen in iron hasmatoxylin preparations. In P. maximus 



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