MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 191 



tlic postei'ior portion of tho hood. From its imicrxation. which i.s derived from the eei'ehrul, 

 pechd. and pleuro-vLsceral gang-lia, the posterior portion of the hood seems to be fairly compar- 

 able with the dorsal portion of the nuchal region of the Gastropoda. 



EYE. 



The eye of the Nautilus is bowl-shaped, to use a rougii comparison, the top of the bowl being 

 closed by a thin membrane which is perforated centrally Ijy a small round hole, while the base of 

 the bowl projects as a short stalk which is attached to the side of the head (Fig. 1. E). The top 

 of the bowl is turned outward. The eye does not seem to be round, but somewhat triangular, 

 the rounded apex tieing directed ventrally. It is 22 millimeters in length (antero-posteriorly), 

 15 millimeters high (dorso-veutrally). and 12 millimeters from the base to the outer side. The 

 stalk of the eye is 9 millimeters in diameter. ))ut only 2 or ?> millimeters long. 



The edge of the eye. is produced into a flange \entrally and laterally, Init not dorsally. The 

 round aperture in the outer face of the eye is usually spoken of as the pupil and lead^ into the 

 cavity of the eye lined liy the retina, there being no lens or any medium of refraction in the eye. 

 The sea waterhas free ingress to or egress from the cavity of the eye. The pupil is about 2 

 millimeters in diameter. From the ventral side of the pupil a groove leads across the face of the 

 eye to its ventral edge. The posterior edge of the groo\e projects over the groove to the opposite 

 side, transforming this into a tubular channel. 



Usually the edges and the center of the face of the eye are slightly raised. lea\'ing a depression 

 between them. Except for a few almost microscopic depressions of the surface, the face and 

 sides of the eye are quite smooth. The outer epithelium of the eye is composed of long, slender 

 ciliated cells. The groove on the face of the eye is lined with a similar epithelium. Von Hexsen 

 suggests that a constant stream of water may be driven through it, keeping the pupil clean, and 

 preventing the entrance of foreign bodies into the eye. 



The membrane forming the outer face of the eye is quite thin; 1 millimeter thick at the 

 edges, it gradually Ijccomes thinner until the pupil is reached, where it is scarcely thicker than 



writing paper. 



The sides of the eye are much thicker than the outer face, and they increase in thickness as 

 they approach the stalk. The dorsal side is al)Out 1.5 millimeters in thickness; the ventral side 

 is double this thickness. The capsule of the eye is composed for the most part of connective 

 tissues; a little muscular tissue is also present. 



In longitudinal section the cavity of the eye is oval with a blunt outer and somewhat 

 pointed inner pole. The wall of the outer portion of the cavity is of an intense l)hick color. 

 This black area is approximately cirt'ular. The side and back walls ar(> of a light gray color, 

 the line of demarkation between the black and gray portions of the wall being very sharp. The 

 diflerence in color is caused by the fact that in the posterior portion of the cavity the retina bears 

 rods which hide the pigment, while the rods are absent on the anterior portion of the retina. 



I follow Hallek's account of the structure of the retina. The retina is about 1 millimeter 

 in thickness at the posterior side of the cavity, gradually becoming thinner as it passes anteriorly. 

 The branches of the optic nerve spread out in a thin fibre layer immediately beneath the retmal 



epithelium. 



The epithelial layer of the retina is composed of two kinds of cells. The first are columnar 

 cells, having a width" equal to about one-eighth of their height. The nucleus lies in the upper 

 end of the fowcr third of the cell. Fine pigment granules lie in the cell above the nucleus, but 



rarelv below it. u i i-i • 



The second kind of retinal cells are much more slender than the first, almost thread-like m 

 shape. The nucleus usually lies at the beginning of the upper third of the cell. The jiigment 

 <rranules are larger than in the broader cells, and often are so large compared with the width of - 

 the cell bodv as to be arranged like a string of beads. They also frequently extend below the 

 nucleus. The broad and line cells alternate regularly. The pigment does not extend quite to 

 the upper end of the cell, nor often into the lower third of the. cell. Thus the pigment forms a 

 dark liaiid which is verv noticeable in sections oi the eye. 



