242 bulletin: museum of comparative zoology. 



1 (Plate 1) is a semi-diagrammatic, longitudinal section of the eye-stalk, 

 which shows the eye retracted. The involution (iol. ta.) in the bottom 

 of which lies the eye, though parallel to the longitudinal axis of the ten- 

 tacle, is eccentric, being dorsal. The large tentacular nerve (n. ta.) runs 

 along the ventral side of the tentacle. In addition to a complicated sys- 

 tem of muscle-fibres (Jbr. mus.), the tentacle contains large masses of 

 unicellular, mucous glands (cl. yl.). The remainder of the tentacle con- 

 sists principally of mesodermal cells supported by abundant connective 

 tissue, among which, here and there, lacunae (lac.) are visible. 



The retracted eye lies at the proximal end of the invagination. Its 

 distal pole is in contact with the basement membrane of the overlying 

 epithelium, which is here unpigmented and translucent. In form the 

 eye approaches a sphere, but its chief axis is a little shorter than its 

 dorso-ventral axis, for the reason that the so-called accessory retina lies 

 in the antero-ventral part of the eye. The result is that the accessory 

 retina causes a protrusion of the eye in that region and increases the 

 dorso-ventral axis abnormally. The shape of the eye in a longitudinal 

 section at right angles to that of Figure 1 can be seen in Figure 2, 

 which is reproduced from a photo-micrograph of a gold chloride prepara- 

 tion. Figure 18 (Plate 2), which is similarly reproduced from a vom 

 Rath preparation, shows the eye cut in a plane parallel to that of 

 Figure 1. From a comparison of Figures 1 and 2 it is evident that the 

 invagination (ivl. ta.) of the tentacle is wider from side to side than 

 dorso-ventrally. The outline of the eye, as seen in Figure 2, is nearly 

 circular, none of the accessory retina being included. The relative posi- 

 tions of the optic and tentacular nerves in the eye-stalk are shown in 

 Figure 19 (Plate 2), dorsal being up in the Figure. 



An axial dorso-ventral section of the eye reveals the following parts : 

 (1) Optic capsule; (2) Cornea; (3) Retina; (4) Lens; (5) Vitreous 

 humor; (6) Optic nerve ; (7) Accessory retina. 



The optic capsule (Plate 1, Fig. 1, cps. opt.) is a connective-tissue 

 sheath enveloping the eye. It is very thin distally, but thicker proxi- 

 mally, where it is continuous with the sheath of the optic nerve (n. opt.). 

 The capsule furnishes a foundation for the attachment of the cellular 

 elements of the eye. 



The cornea and retina together constitute a closed sac, the wall of 

 which is everywhere one cell thick ; the sac is attached to the capsule 

 by structures to be described later. As is well known, this cellular sac 

 is derived by invagination from the tentacular epithelium in the course 

 of the development of the animal. The distal third (approximately) 



