170 



COMPARATIVE ANATOMY 



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 Gastropoda, only found in such Dintocardia as show 

 primitive characteristics, e.g.Haliotidtc, Patcllidc?, 7rocfcidce,DelpJviiw.ilidce, and Stonu/- 

 tiidcc. 



In connection with the claim that JVcc/iVcs (Fig. 145) is the most primitive form 

 among extant CcpJialopoda, it is interesting to find that both its eyes are 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. Optie 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. 



The tentacular eyes of most Guxtro- 



Fio. 140. -Eye of a Pulmonate. 1, J 



Outer, 2, inner cornea ; 3, body epithe- pOttd, CXCBpt those IhotOMn/UI which 



limn ; 4, vitreous body ; 5, retina ; C,, have Clip-like eyes, are of this simple 



ganglion opticum Coptic nerve, character. 



3. The Eye of the Dibranehiate Cephalopoda. 



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

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



kingdom. 



