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 

 r i f ^ 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 (Amphioxus) are of even simpler 

 kind. The so-called " eye " is merely a 



Fig. 1060. Diagrammatic Cross Sec- 11* i 1 



tion through the Head of a Tadpole, to deeply - pigmented spot m the extreme 

 2KV*^*-^S front end of the nerve-tube, and there 



retina and its external pigment-layer; Jg '^ addition a SCHCS of similar but 

 ip.) lens-pit; /., lens; a., an artery; m., 



mouth cavity; ., a nerve; /., pharynx; Smaller SDOtS itt the floOr of the nCrVC- 

 py., pituitary body. 111.111 i 



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 



