170 



COMPARA TIVE ANA TOMY 



CHAP. 



is still a disputed question. In certain cases it has been proved that 

 the pigment in the second kind lies peripherally ; the axis is free 

 from pigment, and may perhaps be considered as the sensitive portion 

 of the cell. In this case, the clear cells would be undifferentiated 

 supporting cells, or secreting cells. The retina is covered, on that 

 side of it which faces the cavity, by a thick gelatinous cuticle, or the 

 whole cavity is filled by a gelatinous body often called a lens. The 

 clear or secreting cells have been thought to yield this gelatinous mass, 

 but there is a tendency to regard them now rather as retinal cells. 



Optic pits are, among the Gastrofoda, only found in .such Diotocardia as show 

 primitive characteristics, e.g. Haliotidu', Patellidcr, TrocJiidiv, DelpMnulidcc, and Stoma- 

 tiida:. 



In connection with the claim that Nautilus (Fig. 145) is the most primitive form 

 among extant Cephalopoda, it is interesting to find that both its eyes ai-e optic pits. 

 Each sensory cell of the retina, i.e. of the epithelial wall of the depression, possesses 

 a cuticular rod projecting towards the cavity, and a layer of ganglion cells is 

 intercalated between the ramifications of the optic nerve and the retina. 



2. Optic Vesicles or Vesicular Eyes. 



Optic vesicles are developed from optic pits both ontogenetically 

 and phylogenetically by the approximation of the edges of the pit, 



which finally fuse. A vesicle is thus 

 formed, over which there is a continuous 

 layer of epithelium (Fig. 146). The 

 outer epithelium is free from pigment 

 over the eye, and is called the outer 

 cornea, while the immediately subjacent, 

 and also unpigmented, epithelial wall of 

 the vesicle forms the inner cornea. The 

 epithelial base of the original depression 

 here again forms the retina ; its cells 

 contain distinct rods projecting towards 

 the cavity of the vesicle, which is filled 

 with a gelatinous mass. The optic 

 nerve usually swells into a peripheral 

 ganglion opticum before reaching the 

 retina. 



Tlie tentacular eyes of most Gadro- 

 poda, except those Diotocardia which 

 have cup-like eyes, are of this simple 

 character. 



Fig. 140.— Eye of a Pulmonate. 1, 

 Outer, 2, inner cornea ; ?., body eijithe- 

 lium ; 4, vitreous body; 5, retina; 0, 

 ganglion opticum ; 7, optic nerve. 



3. The Eye of the Dibranehiate Cephalopoda. 



This is one of the most highly-developed eyes in the whole animal 

 kingdom. It is a further development of the cup-shaped and vesicular 



