494 



SCIENCE. 



[N. S. Vol. XI. No. 274. 



mm. in length, is found in the caves of the 

 Ohio Valley. TyphUchthys is also found 

 in the Ohio Valley but chiefly south of the 

 Ohio river. But a single specimen has 

 been found north of the Ohio and this 

 represents a distinct species. Troglichthys 

 which is found in the caves of Missouri, has 

 been in caves longer than its relatives if 

 the degree of degeneration of its eyes is a 

 criterion. 



Before dealing veith the degeneration of 

 the eye a few words are in order on the 

 normal structure of the organ under con- 

 sideration . 



In the normally developed eye we may 

 distinguish a variety of parts with differ- 

 ent 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 struc- 

 tures, the fibrous or cartilaginous sclera. 



D. The eye itself consisting of: 



1. Parts for transmitting and focusing 

 light ; the cornea, lens and vitreous body. 



2. Parts for receiving light and trans- 

 forming it to be transmitted to the brain ; 

 the retina. 



3. 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 pro- 

 tective and supporting organs, are passive. 



A. In the Amblj'opsidse the skin passes 

 directly over the eye without forming a free 

 orbital rim or lid. The skin over the eye 

 in Chologaster is much thinner than else- 

 where and free from pigment. In the other 

 species of the family the eye has been with- 

 drawn from the surface. In these it lies deep 

 beneath the skin and the latter where it 

 passes over the ej'e, has assumed the struc- 

 ture 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 epigaean 

 salamanders. In Typhlotriton the lids are 

 closing over the eye. The slit between the 

 upper and lower lid is much shorter than 

 normally and the upper lid overlaps the 

 lower. The conjunctiva is still normal. 

 The eye of this species is midway between 

 the normal salamander eye and that of 

 Typhlomolge, in which a slight thinning of 

 the skin is all there is to indicate its former 

 modification over the eye. 



B. The muscles to change the direction 

 of the eye-ball show complete gradations 

 from perfect development to total disappear- 

 ance. 



In the species of Chologaster all the mus- 

 cles are normally developed. In Amhly- 

 opsis the muscles are unequally developed, 

 but one or more are always present and can 

 be traced from their origin to the eye. In 

 Troglichthys, the distal halves of the mus- 

 cles, the parts nearest the eye have been re- 

 placed by connective tissue fibers, t. e., a 

 tendon has replaced part of the muscle. 

 Here we have a step in advance in the de- 

 generation found in Amblyopsis and no in- 

 stance was noticed where all the muscles of 

 any eye were even developed in the degree 

 described. In TyphUchthys the muscles have 

 all disappeared. 



In Typhlomolge the muscles have disap- 

 peared ; in the other sala-manders they are 

 present. 



C. The sclera is differently developed in 

 Chologaster, -and there is but little modifica- 

 tion in the species with more degenerate 

 eyes except that in Amblyopsis and Ti-og- 

 lichthys, wh.ere cartilaginous bands were evi- 

 dently present in the epigsean ancestors. 

 These bands have persisted in a remarkable 

 degree, being much too large for the minute 

 eyes with which they are connected. In 

 Troglichthys they form a hood over the front 



