614 William Patten 



vibrations , irritation of one causing immediate contraction of ali the 

 others , or even of the whole mautle. But, in Pecten , the teutacles 

 are richly supplied with sense hair papillae, while this is not the 

 case with Cardium, where, however, the sensitiveness to lig-ht is highly 

 developed; for variations in the intensity of the latter, caiised by shad- 

 ows, produce lively movements of the tentacles, which may resuit in 

 violent coutractions of the whole animai. After repeated experimente, 

 the irritating effect seems to he dimiuished, so that, finally, even quite 

 deep and sudden shadows may produce only restless or uneasy move- 

 ments of the tentacles, or perhaps no effect at ali. 



Card ita sulcata. 



In fresh specimens, the only material I have examined, may be 

 Seen the orange-red mantle edge, near the orai end of which are five 

 or six large pigment spots, dark brown or nearly black in the centre, 

 but fainter near the periphery. In these pigmented areas, over which 

 the cuticula is specially thickened, one may see numerous, scattered 

 ommatidia, cousisting of four or five dark-colored cells arrauged 

 arouud a single, centrai one, two of which are often situated dose to- 

 gether. The latter may be found on the edge of the dark spots, as well 

 as in the reddish colored area, surrounded, in the latter instance, by 

 pigmented cells of an orange-red color, instead of black. Over the 

 pigmented areas, the cuticula was especially thickened, but not faceted. 



No experiment was made to test the sensitiveness of these ani- 

 mais to light, but I cannot doubt they have this property, since the 

 diffuse ommatidia they certainly possess, appear to be as highly 

 developed as those of Avicula^ which, in comparison with the com- 

 plexity of its organs, has this sensibility developed to a truly wonder- 

 ful degree. 



No statement, that I am aware of, has ever been made concerning 

 the eyes of this genus of Molluscs. 



It is not unworthy of remark, that two genera so closely allied as 

 Cardium and Cardita should have eyes so different from each other. 



Haliotis. 



It was intended, in studying the eyes of this Mollusc , simply to 

 test my observations on Arca^ concerning the nerve endings in the re- 

 tinidia, and the double structure of the retinophorae. 



