Nov., 1902.] Visual Area in Lampsida Ventricosus. 321 



tact between nerve and epithelial cell would be needed for a 

 demonstration, this spot has every appearance of being visual 

 and the writer believes it to be such. 



The mantle was found by repeated experiment to be sensitive 

 to both concentrated rays of light and to shadows. It was not 

 determined whether the visual area was confined to one .spot or 

 generally distributed. 



A section through the eye like .spot shows the epithlinm to 

 be greatly thickened, and nuich more pigmented than the ordin- 

 ar}' epithelial cells. The slightly pigmented epithelium is about 

 15 micromillimeters thick while the eye spot is 60. 



The ordinary epithelium is pigmented throughout about half 

 its length, the remaining basal portion of the cell being taken up 

 with the nucleus, while in the visual cell the nucleus is 15 mm. 

 long and the pigmented area 45 mm. long. 



The outer exposed end of the visual cell bears a well marked 

 corneous like coat while this is almost absent from the surround- 

 ing epithelium. 



In addition to these marked differentations, the basal ends 

 of the visual cells are drawn out into branched processes which 

 are about 15 mm. long. These extend down into the subjacent 

 tissue and supply the place of a rather well defined membrane 

 which exists between the ordinary epithelime and underh-ing tis- 

 sue. While these processes have not been proven to be in con- 

 tact with nerves, yet there is an undoubted visual area here. The 

 modification of the ordinary epithelium into a heavily pigmented 

 epithelinn and this finally into a \isual area as in this form does 

 not involve nearly so radical a change as has gone on in the devel- 

 opement of the pallial e>'es of peden. The corneous lens like 

 body is only a modified cuticle and any cell bearing pigment may 

 be .sensitive to light. 



The presence of .so well defined visual area in one of our 

 fresh water clam is a striking fact since the group as a whole 

 seems to be entirelv insensitive to light. 



