78 



THE ORIGIN OF VERTEBRATES 



is much larger than the left. This difference between right and left 

 indicates a greater degeneration on the left side, and points distinctly 

 to a close relationship between the nerve-masses known as ganglia 

 habenulcB and the median eyes. In my opinion this ganglion is, in 

 part, at all events, the optic ganglion of the median eye on each side. 

 It is built up on the same type as the optic ganglia of invertebrate 



Fig. 32. — Eye op Acilius Larva, with Fig. 33. — Pineal Eye op Ammoccetes, 

 its Optic Ganglion. with its Ganglion Habenula. 



On the right side the nerve end-cells On the left side the eye is drawn as it 



have been drawn free from pigment. appeared in the section. On the right 



side I have removed the pigment from 

 the nerve end-cells, and drawn the eye 

 as, in my opinion, it would appear if 

 it were functional. 



simple eyes, with a cortex of small round cells and a medulla of fine 

 nerve-fibres. Into this ganglion, on the right side, there passes a very 

 well-defined nerve — the nerve of the dorsal eye. The eye itself with 

 its nerve, pn. } and its optic ganglion, g.h.r., is beautifully shown by 

 means of a horizontal section through the head of Ammoccetes 

 (Fig. 31). Originally, as described by Scott, the eye stood vertically 



