232 K. KISIIINOUYE. 



celliiliir atriictiire, tboiiu'h iiccordiu^- t(j J 'at ten (he like-uamed part of 

 Pecten and Canliinii edah is waid to liave been ])roduc('d from cell 

 Invers. The rapetuni covers entirely the lower surface of the clioroid, 

 except at the spots through wliich tlie branches oi (he <jptic nerve 

 enter. If the taj)etuni of I'ccten and Cordimil cdule is cellular in oriu'in. 

 there is in these iiujUtiscs iio organ homologous with the ta])etnni of 

 Cardium midicum. Moreover, the tapetuni of I'ccten and Cardiinii cdtdc 

 is said to be found below the layer of retinophora\ while that oï Car- 

 dram muticum is beh^w the choroid. 



Lastly there conies a layer of pigmented cells (tig. z^, p.). 'Lliis in- 

 vests all the external surface of the above described parts of the eye 

 below the cornea, leaving only a small round area o^er the upper 

 hemispherical portion of the lens or the pupil only. The pigmented 

 layer as a whole is cu|)-shaped or rather urn-shaped. It consists of 

 fiat,. polygonal cells arranged in one layer. The cells at the neck of the 

 urn are smaller and thicker than those elsewhere. ihe ]>it>'ment is 

 black and serves to absorb rays of light which fall obliquely upon the 

 retina. In Pecten the greater part of this function is fulfilled by the 

 iris, and its red pigment layer, probably homologous with the pigment 

 layer of Ccwdimn niuticmii, aljsorbs rays of light fr(3m the lower 

 side alone. As the eyes of Canliuni innticum are destitute of the 

 iris, the pigment layer is well developed. 



The eyes are innervated from the viscero-parietal ganglia. A nerve 

 (tig. 2, n.) runs through the central axis of each tentacle. It is 

 divided into two branches where it touches tlie eye. One branch 

 (fig. 2, /i " ) passes through the ])igment layer near the optic axis and 

 spreads between the choroid and the tapetum, while the other branch 

 (fig. 2, n ' ) passes through the pigment layer at tlie level of the retina 

 from the shell-side and seems to innervate the retina. 



As the numerous eyes of an adult individual are sometimes found 



