322 Introduction to Animal Morphology. 



or a pair of ciliated grooves, one behind each eye, 

 supplied by a nerve arising along with the optic (this 

 organ is smaller in Octopods than in Decapods). 

 Another peculiar sense-organ lies on the inner wall 

 of the funnel, as a flat, whitish eminence whose surface 

 consists of cells containing strongly refracting, rod- 

 like bodies. The organ of hearing is a round sac, 

 in Tetrabranchs lying on, in Dibranchs inclosed in, 

 the head cartilage, and containing one or many 

 (Nautilus) flat or rounded otoliths ; its nerve springs 

 from the pedal ganglion. There is a membranous 

 sac or labyrinth within the cartilage, separated from 

 it in Octopods, but joined to it by processes and sulci, 

 which renders it complex in Decapods. The nerves 

 end in fine rods, or under a thick epithelial " auditory" 

 plate. A ciliated canal traverses the cartilage, 

 whereby the dermal sac has been involuted for the 

 formation of the otocyst. The optic nerve arises from 

 the brain, and forms a large ganglion directly behind 

 the large eye-ball, whose orbit of cartilage is conti- 

 nuous into the sclerotic. Below and behind the eye- 

 ball is a peculiar "white body,"* probably an aborted 

 nerve ganglion, representing the epipharyngeal gan- 

 glion of other molluscs. The anterior wall is trans- 

 parent, and forms a cornea in Loliginidae. 



Investing the eyeball is a silvery lamina (argentea ex/erna), 

 under which is a somewhat cup-like cartilage forming the 

 fundus of the eyeball, and pierced by the branches of the 

 optic ganglion. The thick front edge of this cartilage is 

 called the equatorial ring, and from it longitudinal muscular 



* In development the optic ganglion grows at the expense of the white 

 body, and, according to the researches of Mr. Lankester, the optic nerve 

 and ganglia appear to be developed from the raesoblast. 



