248 proceedings of the academy of [1883. 



November 6. 

 The President, Dr. Leidy, in tlie chair. 



Forty-four persons present. 



A paper, entitled " On the Value of the ' Nearctic ' as one of the 

 Primary Zoological Regions. Replies to Criticisms by Mr, Alfred 

 Russel Wallace and Prof. Theodore Gill,'' b}' Professor Angelo 

 Heilprin, was presented for publication. 



On Visual Organs in Solen. — Dr. Benjamin Sharp called 

 attention to a remarkably primitive form of visual organ that he 

 had discovered in the siphon of Solen ensis and S. vagina (the 

 common '* razor-shell "). 



His attention was directed to the probable possession of visual 

 organs by observing a number of these animals which were 

 exposed in large basins for sale at Naples. A shadow cast by 

 his hand caused the extendi d siphons of the specimens on which 

 the shadow fell, instantly to retract, while those not in the shadow 

 remained extended. Repeating tliis experinient at the Zoological 

 Station at Naples, and being fully convinced that the retraction 

 was due to the shadow and not to a slight jar which ir.ight have 

 been the cause; he was led to examine the siplion more closely, 

 and he also made a seiies of veitical sections for the purpose 

 of very minute stud} . 



When the siphon of a large Solen is cut open and examined, a 

 number of fine blackish brown lines or fine grooves are seen. 

 These are situated between and at the base of the short tentacular 

 processes of the external edge of the siphon. As many as fifty 

 of these little grooves were found to be present in some speci- 

 mens, and some of them were from 1 to 1'.5 mm. in length. 



When a vertical section is examined these pigmented grooves 

 are distinctly seen, and the cells of which they are composed are 

 very diti'erent from the ordinary epithelial cells which cover the 

 more pigmented parts. Tliese latter cells are ordinary columnar 

 epithelial cells witii a large nucleus which is situated near the 

 tunica on which it rests. The pigmented cells are from one-third 

 to one-half longer than those just described, and consist of three 

 distinct parts. The upper part, or that part farthest from the 

 ^wmca, appears perfectly transpaient and takes up about one-ninth 

 or one-tenth of the total length of tlie cell ; this part is not at all 

 affected with the coloring matter which was used in coloring the 

 whole. The second part of the cell is deeply pigmented and con- 

 sequently opaque ; it is filled with a dark brown or almost black 

 granulated pigment; thi-* takes up about one-half of the length of 

 the cell. Below this is the third part cf this cell, consisting of 



