3 i8 



THE CEPHALOPODA 



head and are generally sessile. They are, however, pedunculated 

 in many embryos (Fig. 290, (8), (9)) and in the adult Ta&nius (Fig. 

 253, e) and other Cranchiidae, and also in some Amphitretus (Octopod). 

 The eye of Nautilus (Fig. 293, e) is an open vesicle with a minute 

 aperture (Fig. 6, A) : it is devoid of any kind of refractive apparatus, 

 and its internal retinal wall is pigmented throughout. In the 

 Dibranchia the cavity of the eye is closed, as it is in the majority 

 of Gastropoda, and the ocular globe consists of the same essential 



J3C 



Fio. 283. 



Horizontal section of the eye of Septa, at, argentine integument ; C, external cornea ; ci, 

 ciliary body ; g.o, optic ganglion ; ik, cartilage of the " iris " ; k; Jf, capeular cartilage ; KK, 

 cephalic cartilage ; L, lens ; o, optic nerve ; 1', retinal pigment ; Re, l:i, external and infernal 

 layer of the retina ; IT, white body. (From Lankester, after Hensen.) 



parts, viz. retina, cornea, and crystalline lens, with various accessory 

 parts added, making it a very complex and perfect organ of vision. 

 The ocular globe is applied to the cephal'c cartilage, is sometimes 

 contained in a more or less incomplete orbit formed by a wing- 

 shaped expansion of the cartilage (Sepia), and is provided with a very 

 large optic ganglion (Fig. 283, g.o). In the Dibranchia the retina 

 occupies the inner part of the ocular cavity, and the rods are turned 

 towards the light. The cornea is situated between the two segments 

 of the cuticular body forming the crystalline lens (Fig. 283, L) : 

 above the more superficial and smaller segment of the lens a fold of 

 the integument forms a contractile iris, with a circular (Oigopsida) 



