The Eye of Pecten. By Sydney J. Hickson, B.Sc, 



Scholar of Downing College, Cambridge. (With Plates 

 I and II.) 



The general absence of organs of vision amongst the 

 members of the class Lamellibranchiata meets with a curious 

 and interesting exception in the genera Pectenand Spondylus. 



These genera have long been known 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 paral- 

 lel 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 w^hich are given ofl" 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, ' Miiller's Archiv,' 1840, p. 301, pi. xi. 



' Duvernoy, 'Meraoires de i'Academie de Sciences,' t. xxiv, 1852. 

 * Memoire sur Ic systeme nerveux des Acepliales,' p. 73, pi. ii. 



^ Blanchard, ' Organisation du rcgne animal : Mollnsques Acephalds.' 



* Kerferstein, ' Zeit. fur wiss. Zoolo^ie,' 1863, p. 133. 



* Hensen, ' Zeit. t'iir wiss. Zoologie,' 1865, p. 220. 



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