244 BULLETIN: MUSEUM OF COMPAEATIVE ZOOLOGY. 



ated. In the figure it appears as a pale covering (ivlr. bac.) for the 

 axis. 



Proximally each sensory cell gives rise to a neurite, which passes out 

 of the capsule at the pi'oximal pole of the eye to constitute with its 

 fellows the optic nerve. The optic nerve (Fig. 1, n. opt.) consists of 

 parallel neurites, and is ensheathed in connective tissue, which is con- 

 tinuous with that of the capsule. Within the capsule of the eye the 

 neurites are arranged in loose strands, which run along the inner face 

 of the capsule among the cells of the retina. The strands converge at 

 the optic nerve and pass out in a manner which will he described later. 

 Pieces of strands may be seen in Figure 18, n't. (Plate 2). Distally the 

 strands grow smaller and smaller toward the cornea, as their neurites 

 are distributed to the sensory cells. 



It is not my purpose to describe in detail the lens, which Simroth 

 ('76) has treated more fully than any one else. The fresh lens is a 

 spheroidal, highly refractive, apparently homogeneous body, which one 

 can easily dissect out. If alcohol is added to the preparation, the cen- 

 tral part of the lens grows porous from loss of water. It then shows 

 the compact, peripheral layer and the porous centre which Simroth dis- 

 cusses at length. Sections of the lens show the conditions more clearly. 

 The lens has no special supporting mechanism, such, for example, as is 

 found in the cephalopod eye, but it may be in contact either with the 

 cornea or with the free ends of the longer rods. 



The vitreous humor (Fig. 2, liu. vit.) fills the narrow space between 

 the r*ods and the lens, wherever a space exists, as well as the interstices 

 between the rods. Unfortunately, it becomes brittle in the course of 

 preparation so that it is easily broken into fragments by the knife. In 

 Figure 2 it is less injured than in some cases and its position in the eye 

 is not greatly disturbed. 



Except for differences in the size and form of the parts, the facts thus 

 far stated are common to all pulmonates. In Lirnax, however, there is 

 a remarkable structure at one side of the cornea which was discovered 

 by Henchman ('97) and more recently described by Hesse (:02). The 

 former refers to it as the secondary or accessory retina, the latter as 

 the Nebenretina. Figure 18, rtn. ace. (Plate 2) shows its location in the 

 antero-ventral part of the eye, at the ventral margin of the cornea. It 

 is peculiar in that it contains no trace of pigment, but consists entirely 

 of sensory cells, interspersed among cells which have the form and ap- 

 pearance of corneal cells. I shall describe the accessory retina in detail 

 later. 



