302 PROCEEDINGS OF THE ACADEMY OF [1884. 



siphon within the shell, protection is thus afforded to these 

 delicate organs. 



When we tind forms which are unable to wholly retract the 

 siphon within the shell, the visual cells are confined to grooves 

 at the bases of the tentacles. In the rapid withdrawal of the 

 siphon through the sand, in cases of danger, we can easil}' see 

 that the sharp particles would irritate any delicate org'an, and 

 protection must be afforded to them. Now, the possession of 

 sight at the only exposed portion of the animal, would be of the 

 highest value, in the struggle of life, to its possessor, if, when a 

 shadow, like that of a rapacious fish, is thrown upon the organ 

 of sight, a rapid retraction will save it from being nipped off. 



In Solen vagina and S. ensis, the former being 

 the species on which I first satisfactorily proved 

 the existence of a visual sense, ^ we find the cells 

 have become much more developed, as is seen in 

 fig. 2 (this being drawn to the same scale as that 

 of fig. 1), but are essentially on the same plan as 

 those in Ostrea. These cells line deep grooves at 

 the bases of the tentacles and are found nowhere 

 visual' ceiT of else, thus being amply protected from any injur3\ 



Sole7i vaqina. c, mi i • ^.i -in v. 



cuticle; p, pig- The nervcs supplying these visual cells are prob- 

 ment ; n, nucleus. ^^^^ ^j^^ ncrves of general sensibility, perhaps 



somewhat specialized. 



The remarkable organs of Pecten and Spondylus, I will not 

 here consider, as they throw no light on our immediate subject, 

 and have been considered by me elsewhere.^ 



In passing next to a higher group, the Gastropoda, from which 

 the Lamellibranchiata have probably degenerated, marked steps 

 in advancement are to be noted. 



In Patella we find that the pigment spots, or visual organs, take 

 their morphological position, namely in the oral end of the body, 

 and consist of a single pair in the base of the broad tentacle. 

 More than a single pair of eyes are not found in the Gastropoda.' 



' Sharp, B., On Visual Organs in Soleii. Proc. Acad. Nat. Sci. Phila., 

 Nov. 6, 1883, p. 248 ; also, On the Visual Organs in Lamellibranchiata. 



'' Sharp, B., On the Visual Organs in Lamellibranchiata. 



■' The adajtive dorsal eyes of Onchidiiim form an exception, but the 

 normal pair of cephalic eyes are present. See Semper, Carl, Ueber Seh- 

 organe von Typus der Wirbelthiereaugen am Riicken von Schnecken. 

 Wiesbaden (Kreid.l), 1877. 



