On the Visual Organs in Lainellibranchiata. 461 



edge , while uo trace of this second set of cells is present in the forms 

 with Siphons. 



These latter cells are filled with a granulated protoplasm , which 

 seems to take less of the staining fluid than that of the pigmented cells, 

 so that a contrast between these two sets of cells is formed. The nuclei 

 are spherical and are placed in the broad ends of the cells, between the 

 tapering ends of the pigmented cells. 



Those pigmented cells, as well as the other cells that line the wall 

 of the branchial Chamber , I feel convinced are ciliated , and I am in- 

 clined to the same opinion as regards the corresponding one in Ostrea^ 

 but I could not proove it entirely to my satisfaction , and consequently 

 will not State that such is the case. Those cells situated in the folds 

 then do not seem to have any trace of cilia. 



Dreissensia polymorpha. Here also I am sorry to say that I have 

 no experimental evidence of vision, but the anatomical structure agrees 

 so exactly with other forms, on which I have experimented that I do 

 not hesitate to say, that A'ision is present. 



The mantle in this species is closed and with only a small byssal 

 orifice ; the siphon is only indicated by two specialized openings at the 

 anterior part of the animai, in which considerable pigment is deposited. 



When sections of these simple siphons are made, large and clearly 

 defined cells are seen , which are somewhat smaller but much broader 

 than those foiind on the mautle-edge oi 3Iytilus. On those cells, in 

 which pigment is fouud, a clear and refractive cuticle is seen ; this cu- 

 ticle is greatly developed and exceeds, in fact, that of any other Lamelli- 

 branch that I have examined (fig. 16 c). The nuclei are long and large 

 and the pigment is deposited between them and the cuticle fig. 16). 



These pigmented cells are deposited in a groove , which extends 

 around the mouths of the siphons. 



Farn. Arcidae. 



Area incongrua was found to possess no trace of a deposit of pig- 

 ment in the mantle. Will, however, states that Area has complicated 

 eyes on the mantle , but I do not know what species he examined. In 

 Area americana there are crescent-shaped deposits of pigment at regulär 

 intervals a little above the mantle-edge on its exterior surface, no other 

 trace of visual Organs are present. 



Pectuncuius sp. (?) The fact of this form possessing highly developed 

 Organs of sight , is known to me only through the works of Will, 

 Siebold and Gegenbaur ; Siebold stating that numerous brownish- 



