THE EYE OP PECTEN. 105 



with an axial neurofibril. The retina is of the inverted type. 

 The eyes are not cephalic, but occur on the mantle-lobes. 



There is no ground whatever for placing the Pecten eye in 

 the same class as the vertebrate eye, for the resemblance is 

 very superficial, and though the retina is inverted in both 

 cases this has been produced in very different ways. If we 

 consider Biitschli's observations as correct the retina of the 

 Pecten eye has been formed from an invagination of the ecto- 

 derm, which forms a closed vesicle cut off from the surface. 

 The distal wall of this gives rise to the retina, and the 

 proximal to the pigment layer. 



Amongst invertebrate eyes that of Spondylus is the only 

 one that can be safely homologised with the Pecten eye. 

 The structure of these organs is identical but for one point, a 

 layer of cells in Spondylus takes the place of the non-cellular 

 septum of the Pecten eye. The eye of Cardium can also be 

 homologised, though with less certainty. There is a cellular 

 lens, an inverted retina with two series of recipient cells, and 

 also layers corresponding in position to the tapetum and pig- 

 ment layer of Pecten. There is, however, another layer (the 

 choroid) interpolated between the retina and tapetum, which 

 may be taken as equivalent to the interstitial cells of the 

 Pecten eye. 



These are, so far as I am aware, the only vesicular eyes 

 occurring in the Lamellibranchiata. 



In the highly organised cephalopod eye we do not meet any 

 resemblance to the Pecteu eye, except that the visual cells 

 bear rods with an axial neurofibril like thesfe recipient struc- 

 tures in the latter. There is a single layer of recipient cells 

 directed towards the light, and the lens is not cellular and 

 arises quite differently from the lens of the arthropod eyes. 



Amongst the Polychgeta there are some highly organised 

 visual organs, in particular those of the Alciopina, ex. Alciopa 

 and Vanadis, and the large and complex organs of these 

 forms have been studied in detail by Greeff and Hesse. The 

 eye takes the form of a closed vesicle as in Pecten, the free 

 pole being formed by a cellular cornea, a continuation of the 



