THE EYE OF PECTEN. 63 



present on the upper side of the eye-stalk, and there is really 

 a longitudinal baud present, exactly similar (though not so 

 definite) to the one on the corresponding side of the tentacles. 

 The pigmented area bounding the cornea was termed the 

 "iris" by Patten. Since, however, as described above, these 

 pigmented cells extend far down the eye-stalk on both sides, 

 it is difficult to make any division into regions or to define a 

 boundary. If, moreover, the physiological action of the iris 

 were considered solely to be that of a diaphragm, keeping 

 out oblique rays, the name might perhaps be applied, but, as 

 Rawitz pointed out, there is no proof whatever of this area 

 being capable of contraction with diminution of the "pupil," 

 and since this region is not to be homologised with the 

 vertebrate structure of the same name it is better to use the 

 term pigment-mantle (PL 6, fig. 1, P. man.) if a special 

 one is necessary. Patten considered that the "pupil" could 

 be diminished to almost half its previous diameter (p. 571), 

 but I have been unable to find any trace of this under natural 

 conditions, nor do any other authors appear to have been 

 more fortunate. The same writer states that on the shell 

 side even in fully formed eyes the pigment may sometimes be 

 absent so that a colourless fissure is left — termed by him the 

 "choroid fissure " (p. 578). I have not seen this in any eye 

 examined, and fail to find any references confirming the 

 statement of its existence. 



The pigmented epithelial cells pass suddenly into the 

 transparent cells of the cornea (PI. 6, fig. 1, Co.), through 

 which is seen in the living specimen the silvery glance of the 

 subretinal structures. In P. maximus the depth of the tall 

 epithelial cells may decrease slightly in one or two cells, and 

 then the next is much lower and completely free from 

 pigment. Sometimes, however, the decrease in height takes 

 place after the pigment becomes absent. 



The nuclei take up again a central position or a position 

 nearer the base in the corneal cells, but there are certain 

 exceptions which will be considered later. The cells are 

 hexagonal in surface view and are much flatter than those of 



