46 NERVOUS SYSTEM AND SENSE-ORGANS 
are bright-red in colour. Their presence is possibly in relation 
to the fact that some species are active swimmers. 
Before speaking of the camera eyes of Vertebrates, it may be 
well to mention certain simpler visual structures which are found 
in some of the most primitive members of that group. In the 
tadpole larva of a Sea-Squirt there is a simple cup-like direction- 
eye formed by thickening of the wall of the brain, and projecting 
into that organ (see fig. 1049, p. 38). Since the larva is transparent 
light-rays are able to reach it. The adult condition results from 
a remarkable series of modifications (see vol. iii, p. 421), which 
include simplification of the nervous sys- 
tem with loss of the brain-eye and brain- 
otocyst. The only compensation for this 
loss of vision consists in the appearance 
of a circlet of pigmented eye-spots round 
the openings by which currents of sea- 
water enter and leave the body. 
The visual organs of the transparent 
Lancelet (Amphzoxus) are of even simpler 
kind. The so-called “eye” is merely a 
Fig. 1060.—Diagrammatic Cross Sec- . 5 
tion through the Head of a Tadpole,to Geeply- pigmented spot in the extreme 
Ilustrateche Develepmentof she Syes, front end af the netve-tubée, and there 
enlarged. , Fore-brain; 7. and 71., 
retina and its external pigment-layer; i¢ in addition a series of similar but 
%., lens-pit; 2, lens; @., an artery; 72., 
mouth cavity; 7.,a nerve; ##., pharynx; smaller spots in the floor of the nerve- 
py» pituitary body. ] : 
tube behind the head-region. 
The facts just mentioned prepare us for the statement that the 
ordinary camera eyes of Fishes and still higher Vertebrates are 
partly derived from the brain, and in this they differ from the 
camera eyes of Invertebrates, which are of epidermic nature. Two 
stages in the development of the Vertebrate eye are represented 
in fig. 1060. From either side of the fore-brain of the embryo an 
optic vesicle grows out towards the ectoderm, in which a corres- 
ponding pit makes its appearance. The end of the vesicle becomes 
as it were pushed in to form a double-walled optic cup, of which 
the inner and thicker layer is destined to produce the greater part 
of the retina, or sensitive eye-screen, while the outermost pigmented 
layer of this is derived from the outer part of the cup. The 
external ectodermic pit closes, and is pinched off as a vesicle, which 
lies in the optic cup (see right-hand side of figure), and ultimately 
thickens into the lens. The stalk of the optic cup becomes the 
