THE .Si:\irLE EYE 



145 



invaginating to line a volummous cul-de-sac extending far posteriorly. Over this 

 the transparent surface epithelium forms a " cornea", sometimes, in Myopsidse, 

 forming a continiiovis layer in which case the cul-de-sac (the " anterior chamber ") 

 is filled with an " aqueous humour " (cuttlefish. Sepia ; squid, Loligo), some- 

 times, in CEgopsidae, perforated by a hole so that the cavity is flushed by seawater 

 (Octopus). The iris is supported by a plate of cartilage and both it and the 

 ciliary body are provided with contractile muscular tissue. The pupil is rect- 

 angular in shape and actively contractile and there is an efficient accommodative 

 mechanism ^ (v. Hess, 1909) ; while covering the iris and extending some distance 



Sepia 









is 



Fig. 115. — The Retina of the Octopus. 



The retina is composed primarily of a single layer of visual cells with 

 rod-like terminations, r, and nuclei, n. Between the rods and the cell-bodies 

 there is a dark line of pigment, p, and at the proximal extremities of the rods 

 a layer of protective pigment, pp. Most externally there is a layer of nerve 

 fibres, /, with ganglion cells. The white line underneath the pigment is an 

 artefact at the site of a supporting membrane ( X 150) (froin a specimen of 

 J. Z. Young). 



posteriorly, is a silvery membrane of pavement epithelium which glitters and 

 shines like mother-of-pearl (Figs. 116-17 ; Fig. 192). 



The retina itself is coiTiprised in the main of visual cells sujDported by two 

 limiting membranes — an internal membrane lining the cavity of the vesicle and 

 an external membrane dividing the retina transversely into two (Fig. 115). The 

 visual cells are made up of two elements, a rod -like termination and a cell -body. 

 The rods lie between the two membranes in palisade arrangement ; they are 

 constricted as they pierce the external membrane, proximal to which lie the 

 cell-bodies with their nuclei, the visual pathway being continued by nerve 

 fibres running in an optic nerve to an optic ganglion.- Prince (1956) described 



■)90. 



p. 52 



Loligo -^^ - 



Octopus 



S.O. — VOL. I. 



