Oo 
mm. in length, is found in the caves of the Ohio Valley. Typhlichthys is 
also found in the Ohio valley but chiefly south of the Ohio River. Buta 
single specimen has been found north of the Ohio, and this specimen repre- 
sents a distinct species. Troglichthys, which is found in the caves of 
Missouri, has been in caves longer than its relatives, if the degree of the 
degeneration of its eyes is a criterion. 
Before dealing with the degeneration of the eye a few words are in 
order on the normal structure of the organ under consideration. 
In the normally developed eye we may distinguish a variety of parts 
with different functions. These are: 
A. Organs for protection like the lid and orbits. 
B. Organs for moving the eye to enable it to receive direct rays of 
light. In the cold-blooded vertebrates these consist of four rectus muscles 
and two oblique. 
C. Organs to support the active structures, the fibrous or cartilagi- 
nous sclera. 
D. The eye proper, consisting of: 
1. Parts for transmitting and focusing light; the cornea, lens and 
vitreous body. 
2. Parts for receiving light and transforming it to be transmitted to 
the brain; the retina. 
8. A part for transmitting the converted impression to the brain; the 
optic nerve. 
Some of these, as the muscles, retina and optic nerve, are active, while 
others, the dioptric, protective and supporting organs, are passive. 
A. In the Amblyopsidae the skin passes directly over the eye without 
forming a free erbital rim or lid. The skin over the eye in Chologaster is 
much thinner than elsewhere and free from pigment. In the other genera 
of the family the eye has been withdrawn from the surface. In these it 
lies deep beneath the skin, and the latter, where it passes over the eye, 
has assumed the structure normal to it in other parts of the head. 
In the salamanders we have a perfect gradation in the matter of the 
eye lids. In Spelerpes a free orbital rim is present in every respect like 
that found in epigzean salamanders. In Typhlotriton the lids are closing 
over the eye. The slit between the upper and the lower lid is much 
shorter than usual, and the upper lid overlaps the lower. The con- 
junctiva is still normal. The eye of this species is midway between the 
3—Se’ence. 
