On the Visual Organs in Lamellibranchiata. 467 



tentacles nor pigmented grooves. I do not know the habits of this form 

 and never saw it alive , but from the form of the siphon and absence of 

 pigment I doiibt if it has any power of vision. 



Besides the pigmented cells were found other peculiar cells scat- 

 tered here and there through the epidermis to which I will merely call 

 attention. These I have represented in fig. 10; they are clear cells 

 situated between the columnar cells of the epidermis, the nuclei, situ- 

 ated near the tunica seem to be crescent-shape , this however may be 

 due to the action of reagents — these cells are tolerably numerous and 

 could be found in all the specimens that I cut. 



Summary. 



In all the forms that I have examined , Spondylus and Pecten pos- 

 sibly excepted , I find that the seat of vision rests in a number of pig- 

 mented epithelial cells , which have at the free end a fine homogeneous 

 cuticlC; which is highly refractive. Although I could find no evidence 

 that nerves were distributed to these cells , it is reasonable to suppose 

 that they are supplied with nerves as are the cells of the general epi- 

 thelial surface. Now when we compare these cells with those found in 

 the eyes of Patella , situated at the base of the tentacles , we find here 

 a similar state of affairs. As is shown by Fhaisse *, the eye of Patella 

 is a simple invagination of the epidermis , forming a sac which is 

 open exteriorly, and allows free access to the surrounding medium, 

 water. Fkaisse could find no evidence of nerves being distributed to 

 this organ. The pigmented cells which liued this invagination were 

 supplied with nerves that were distributed to the general epithelium as 

 was the case with the Lamellibranchiata. that I examined. The pig- 

 mented cells, which make up the eye of Patella (fig. 12), have es- 

 sentially the same form as those found in the Lamellibranchiata. Now 

 in the Lamellibranchiata we have even a simpler form of visual organ, 

 than that of Patella , for in Patella we have the organs specialized in 

 number, two, and Situation — in the base of the tentacles at the anterior 

 or oval end of the beast. In the Asiphonata the vision is very general- 

 ized — patches of pigmented cells scattered at large at that portion of the 

 animai most exposed to light and danger. A somewhat higher grade is 

 Seen in the Siphotiata, where the pigment is more localized in the lo wer 



1 P. Fraisse, Über Molluskenaugen etc. 1881, 



31* 



