On the Visual Organs in Lamellibranchiata. 463 



Among the columnar epithelial cells of the siphon are foimd pig- 

 mented ones scattered about in irregulär groups, these are especially 

 numerous iiear the external orifices of the siphon. The pigmented cells 

 seem to be of the sanie size as those, which coutain no pigment , and 

 both are covered with a transparent covering, a euticle. 



The pigmented cells of Venus are by far the longest of any that I 

 have examined (fig. 5) . In shape they are similar to those of Myfilus, 

 the euticle however is much tbicker in proportion. In Vemis however 

 we do not have any of the cells that lie between the tapering ends 

 of the cells. A space is consequently left here, which is due probably 

 to the contraction of the protoplasm , the pigmented part of the cell 

 being more dense, does not contract so much. 



The nuclei of these cells are very long and narrow and more or 

 less surrounded with the pigment (fig. 5) . 



Venus is another of the forms in which Will found very complex 

 Organs of sight. I however bave sought in vain for such organs : but I 

 can assert that the animals perceive shadows , which pass over the ex- 

 tended ends of the siphons and they rapidly contract them. 



Venus verrucosa is another species, that I have had opportunity to 

 examine, and found the end of the siphon much pigmented and dark 

 pigmented grooves at the bases of the tentacles. 



In Tapes decussata well developed pigmented spots were found 

 at the extremity of the siphons. 



Farn. JPetricolidae. 



Petricola 2)holadifor7ms. Through the kindness of Mr. Ford, I have 

 been able to examine several fresh specimens of this species. The 

 siphons are large, and the branchial and anal siphons are not joìned 

 at the extremity. The ends of the siphons are more or less pigmented 

 at their external openings, these being more on the branchial than anal 

 siphon. 



When sections are made the cells forming the pigmented spots 

 are found to be essentially the same as the pigmented cells already 

 described. These cells dififer, however, in the fact that the pigment is 

 more or less collected at that part of the cell nearest the base, that is, 

 where the cell is attached to the tunica. Here the pigment seems to be 

 deposited arouud the nucleus leaving the clear granulated protoplasm 

 between the pigment and the transparent, homogeneous cuticula. The 

 pigmented epithelial cells as a rule are placed at the base of slight 

 folds, thus being more or less protected. 



