On the Visual Organs in Lamellibianchiata. 465 



rounded by tentacles , which are more distinct than in Venus merce- 

 naria ; there are three or four rows of tentacles the smallest outside and 

 the largest inside , the outer ones measuring about 1 mm in length, 

 while the inner are about twice as long (2 mm) . 



At the base of the inner and longer row of tentacles is seen an in- 

 tensely black band , which passes around on the internal wall of the 

 siphon, and is present in both the anal as well as the branchial siphon ; 

 at the basis of the other tentacles are seen little lines of black pigment. 



When a longitudinal section of the wall of the siphon is made and 

 somewhat magnified, we have a figure as is represented in fig, 6. At a 

 we have a section of the black band lying at the base of the internal 

 row of tentacles ; c is one of the external tentacles showing a patch or 

 groove of pigmented cells at each side of the base. 



The substance of the siphon is seen made up of muscular fibers 

 and connective tissue between the bundles of which are seen the lacu- 

 nae (fig. G l). 



When the patches of pigmented cells are more strongly magnified, 

 we find that they are of greater length than the epithelial cells, which 

 are not pigmented (fig. 7). We find that, as in the case of Mactra^ the 

 pigmented cells are concentrated iuto groups, where they are protected 

 as at the base of the tentacles , or inside the siphon , but few are ex- 

 posed at the external wall. 



The cutìcula which extends over the cells, is thickest on those 

 cells, that are farthest from the edges of the groove or patch of cells 

 (fig. 7) , it gradually dimiuishes in thickness , so that none is present, 

 where the unpigmented epithelium is reached. The pigment seems to 

 he thickest, or most opaque, in the center of each celi, so as to give 

 the cells the appearance of containing rods (fig. 7) . 



When the pigmented cells are examined with a high power, we 

 find that they are essentially the same as those described for other 

 species (fig. 8) , save that the pigment instead of being dark brown is 

 jet black in color. 



Fam. Solenidae. 



Solen vagina. This is the form by which my attention was first 

 directed to investigate the visual organs oi La?nellibra?icMafa, and with 

 which I made the most extended and complete experiments , leaving 

 not the shadow of doubt, that visual power existed in these animals. 



A preliminary account was published on November 6, 1883 1, 



' B. Sharp, On Visual Organs in Sohn. Proc. of Acad. of Nat. Se. of Phila- 

 delphia. 1883. p. 248. 



Mittheilungen a. d. Zoolog. Station zu Neapel. Bd. V. 31 



