35 G COMPARATIVE ANATOMY. 



cartilage of tlie iris (ik). Outside and behind this optic capsule is the 

 ganglion of the optic nerve, in the periphery of which there is a 

 whitish organ (w) which projects more or less forwards. Behind 

 this there is a longitudinal layer of muscles, and lastly, a silvery 

 membrane (argentea externa) (ae), which reaches as far as the 

 edge of the pupil, and invests the bulb on its inner face, or 

 the one turned towards the above-mentioned cavity. Internally 

 to it there is the argentea interna. The bundles of nerves 

 which arise from the ganglion (<jo) behind the cartilaginous cap- 

 sule, pass to the retina by a large number of pores in it ; 

 the retina lies within the cartilaginous capsule, and is con- 

 tinued forwards as far as the edge of an organ which carries the 

 lens. This retina is formed of essentially the same layers as the 

 same portion of the eye of the Gastropoda ; an internal (Bi) portion, 

 which contains the perceptive apparatus, is separated from the 

 external part (Be) by a layer of pigment (P). A layer of connec- 

 tive tissue extends inwards to the lens (L) from the layer of muscular 

 fibres ; this completely separates the eye into two parts, an anterior 

 smaller and a posterior larger one ; these unite to form an oval 

 body, the long axis of which corresponds to the optic axis. There 

 are epithelial thickenings on the anterior, as well as on the posterior, 

 surface of this layer of connective tissue ; they unite to form a 

 system of lamellas, which invests the edge of the lens, and is known 

 as the " ciliary body " (ci) (corpus epitheliale). The cavity behind 

 the lens is filled up by a fluid. 



Hensen, Zeitschr. f. wiss. Zool. Bd. XV. 



[Lankestek, E. Kay, Develt. of Cephalopoda. Quart. Journ. Microsc. Sci. 1875.] 



Auditory Organs. 

 § 274. 



It is possible, apparently, to derive the organs, which seem to be 

 auditory in the Mollusca, from the vesicles or otocysts which we met 

 with iu the Vermes. They contain otoliths, and 

 their inner wall is provided with nervous end- organs, 

 which are formed by modified epithelial cells. These 

 cells are derived from the ectoderm, for the otocyst 

 itself is formed from that layer ; this has been ob- 

 served in the Gastropoda. What we know of the 

 Fig. 189. Audi- development of the ear in the Cephalopoda agrees 

 tory organ of with this statement. 



Oyolas. c Au- ^he auditory nerve is generally given off from 



eC^tedepithei tno cerebral ganglion. In its most primitive form 

 lialcells. o otolith the auditory vesicle is attached to this ganglion, and 

 (after Leydig.) it is only when it is separated from it that we are 

 able to make out a true auditory nerve. The otocyst, 

 however, varies greatly in position ; thus, it may be attached to the 

 pedal ganglion; but even then the auditory nerve can always be 



