RELATION OF MEDIAN TO LATERAL EYES 357 



in Pecten resemble the columnar pigment cells of the ommatidia in the 

 eyes of Area, and they indicate a stage in the development of the eye 

 previous to the differentiation of specialized visual cells from pigment 

 cells. A comparison of the two types of eye also indicates the manner in 

 which the eyes of Pecten can be developed from a simple type such as that 

 of Area by invagination of the superficial central area of modified epithelium 

 into the subjacent mesodermal layer which forms the central core of the 

 tentacle. This inference is confirmed by the work of Kishinouye on the 

 development of the marginal or mantle-eyes of Cardium nuticum (cockle), 

 which when fully developed closely resemble those of Pecten, but differ 

 in the existence of a " choroid " layer between the retina and tapetum 

 and by the mesoblastic origin of the pigment layer (Fig. 243). According 

 to the description given by Kishinouye (1894), Dot h right and left mantle 

 edges are beset with dark-brown almost black pigment. They unite at 

 the posterior end of the shell and form a triangular pigment area sur- 

 rounding the siphonal openings. Over this area the right and left valves 

 of the shell do not meet closely, but leave a rather wide gap. In this 

 triangular interval the tentacles are arranged in irregular rows round the 

 siphonal apertures. The larger and longer, which are about 100 in number, 

 bear the eyes. They are bent away from the siphonal openings and each 

 of them has a long band of black pigment on the siphonal side, i.e. the 

 side exposed to the light. The eye appears as a black spot on the siphonal 

 side of the tip of the tentacle, opposite the position of the eye in Cardium 

 edule. A vertical section through the adult eye shows superficially a thin 

 layer of pavement epithelium ; a central area of this epithelium, the 

 " cornea," is unpigmented and clear ; this is surrounded by a zone of 

 pigmented cells continuous externally with the general epithelium cover- 

 ing the tentacle. Beneath the cornea is a lens which is composed of 

 flattened ectodermal cells. Its vertical diameter is considerably greater 

 than the transverse diameter, and it is slightly constricted in the centre. 

 In contact with the lower pole of the lens is a bilaminar, cup-shaped 

 retina which consists of a superficial stratum of columnar cells, the outer 

 deeper ends of which have a rod-like structure, whereas the inner segments 

 of the cells, which are in contact superficially with the lens, are clear and 

 contain a vesicular nucleus. The outer superficial stratum of the double- 

 layered retinal cup is formed by a layer of cubical epithelial cells, which 

 is continuous, at the margin of the cup, with the layer of visual cells. 

 Kishinouye compared this layer with the outer layer of hexagonal pigment 

 cells of the vertebrate retina, and speaks of it as the " choroid " layer. 

 It is covered externally by a non-cellular layer or tapetum and a thin 

 stratum of mesoblastic cells containing pigment. Both of these layers are 

 pierced by the fibres of the optic nerve, which is connected with the 



