THE EYE OF PECTEN, 
443 
The Eye of Pecten. By Sydney J. Hickson, B.Sc., 
Scholar of Downing College, Cambridge. (With Plates 
XXXIV and XXXV.) 
The general absence of organs of vision amongst the 
members of the class Lamellibranchiata meets with a curious 
and interesting exception in the genera Pecten and Spon- 
dylus. 
These genera have long been knowm to possess a great 
number of eyes of considerable complexity, situated on the 
border of the mantle. The number of these eyes varies 
considerably in different individuals, ranging in the genus 
Pecten from eighty to one hundred and twenty. Their 
position also varies ; for, although they are always situated 
on the border of the mantle, yet sometimes they are placed 
at equal distances from one another, and sometimes they are 
clustered together in certain localities. 
Notwithstanding this indefinite element, both in their 
number and position, which might be expected to run 
parallel with a primitive and simple organisation, their 
anatomy is exceedingly complicated, and exhibits all the 
most important structural elements of the eyes of the higher 
Vertebrata. 
The earliest investigations into the anatomy of Pecten’s 
eye are those of Krohn,^ who gives a drawing of the course 
of the optic nerve. This drawing is copied in many of the 
subsequent papers on the subject by other investigators, and, 
as far as it goes, is correct. Duvernoy,^ in his description 
of the nervous system of the Pectens, gives a short descrip- 
tion of the anatomy of the eye. This paper, however, is 
chiefly valuable for the excellent figures and descriptions 
of the distribution of the nerves in the mantle, and the 
filaments which are given off from the main trunks of these 
to supply the tentacles and the eyes. 
The researches of Blanchard^ and of Keferstein^ which 
followed did not add very much to our knowledge on this 
subject, and it was not until 1865 that any careful histo-^ 
logical inquiries were carried on. It was Hensen*’ who first 
' Krohn, ‘ Muller’s Arcliiv/ 1840, p. 301, pi. xi. 
- Duvcrnoy, ‘Menioires de I’Acadeinic de Sciences,’ t. xxiv, 1853. 
‘ Mernoire sur le systeme uerveux des Acephales,’ p. 73, pi. ii. 
^ Blanchard, ‘ Organisation du regne animal : Mollusques Acephales.’ 
Kcifcrstein, ‘Zeit. fiir wiss. Zoologie,’ 1803, p. 133. 
* lienseu, ‘ Zeit. fiir wiss. Zoologie,’ 1865, p. 220. 
