564 GEOEGINA SWEET. 



are extremely well developed — both being concerned with 

 some function not connected with the power of vision. 



3. Eye muscles — abnormal in position, structui'e, and very 

 variable in development. Their usual nerve-supply is absent, 

 its place being taken by a branch of the ophthalmic nerve. 



4. Sclerotic and cornea not distinguishable from one 

 another, nor from the choroid. 



5. Lens absent always. 



6. Vitreous body is practically absent, even in the least 

 degenerate forms. 



7. Pigment layer of retina very thick distally, and thinner 

 proximally. 



8. Pupil absent, iris being only represented by a few 

 elono-ate nuclei in some cases. These also are absent in 

 other eyes observed. 



9. Eods and cones are absent, simplified nuclear and 

 molecular layers being present rarely — otherwise the retina 

 is represented by an undifferentiated mass of cells. 



10. Optic nerve-fibres. Probable remnants are found only 

 in one case within the eye. They cannot be traced towards 

 the brain except rarely, and for a short distance by its 

 connective-tissue sheath. 



B. Comparisons. 



To summarise the most important of these. 



1. Aquatic Forms. — Of these Troglichthys and Ambly- 

 opsis are the most nearly comparable in structure with that 

 of Notoryctes (higher stages). Typhlichthys and Typlilo- 

 mol""e have reached a similar condition in many points, but 

 the muscles in each are more degenerate, and there is no 

 conjunctival sac or lachrymal gland, and in Typhlichthys the 

 nerve can always be traced to the brain. 



The eye of Proteus is, in all points, more highly developed 

 than that of Notoryctes, the retina only being similar to that 

 of the higher stages of this form. 



2. Burrowing Forms. — In Siphonops we find most of 

 the parts such as the muscles, iris, and lens well developed, 



