The epiblast lining the posterior region of the vesicle gives rise to the retina, that lining the anterior region to the ciliary body and processes. It is important to notice that the condition of the eye just before the above pit becomes closed is exactly that which is permanent in Nautilus (_vide_ fig. 122 A). After the pit has become closed a mesoblastic layer grows in between its wall and the external epiblast. The lens becomes formed in two independent segments. The inner and larger of these arises as a rod-like process (fig. 124) projecting from the front wall of the optic vesicle into the cavity of the vesicle. It is a cuticular structure and therefore without cells. By the deposition of a series of concentric layers it soon assumes a spherical form (fig. 125, _hl_). The condition of the eye, with a closed optic vesicle and the lens projecting into it, is that which is permanent in the majority of Gasteropods (_vide_ fig. 122 B). At about the time when the lens first becomes formed a fold composed of epiblast and mesoblast appears round the edge of the optic cup (fig. 124, _cc_), and gives rise to a structure known in the adult as the iris. Shortly afterwards this becomes more prominent (fig. 125, _if_), and at the same time the layers of cells of the ciliary region in front of the inner segment of the lens become reduced to the condition of mere membranes (fig. 125 B); and in front of them the anterior or outer segment of the lens becomes formed as a cuticular deposit (fig. 125 B, _vl_). At a still later period a fresh fold of epiblast and mesoblast appears round the eye and gradually constitutes the anterior optic chamber (_vide_ fig. 122 C, _Co_). In most forms this chamber communicates with the exterior by a small aperture, but in some it is completely closed. The fold itself gives rise to the cornea in front and to the sclerotic at the sides. At a later period another fold may appear forming the eyelids (fig. 122 C, _Pal_). [FIG. 124. TRANSVERSE SECTION THROUGH THE HEAD OF AN ADVANCED EMBRYO OF LOLIGO. (After Bobretzky.) _vd._ oesophagus; _gls._ salivary gland; _g.vs._ visceral ganglion; _gc._ cerebral ganglion; _g.op._ optic ganglion; _adk._ optic cartilage; _ak._ and _y._ lateral cartilage or (?) white body; _rt._ retina; _gm._ limiting membrane; _vk._ ciliary region of eye; _cc._ iris; _ac._ auditory sack (the epithelium lining the auditory sacks is not represented); _vc._ vena cava; _ff._ folds of funnel.] Auditory organs. A pair of auditory sacks is found in the larvæ of almost all Gasteropods and Pteropods, and usually originates very early. They are placed in the front part of the foot, and on the formation of the pedal ganglia come into close connection with it, though they receive their nervous supply in the adult from the supra-oesophageal ganglia.