THE EYES OF TROGLICHTHYS. 



129 



The oblique muscle is attached by a tendon to the face of the eye opposite 

 that of the attachment of the rectus (fig. 46 a, msc). In the best-developed condi- 

 tion it was found to be but 9 /a in diameter, taking its origin at a point on the level 

 of the lower surface of the olfactory nerve where the latter pierces the ethmoid 

 and 160 /x laterad from it. The muscle itself is in this instance about 200 fj. in 

 length and is attached to the eye by a tendon of equal length. The rectus in the 

 same individual is 208 /a long. 



In all the cases enumerated above the muscles of the opposite side were not 

 nearly so well developed. In the one with the well-developed rectus the oblique 

 was indistinct, while in the one with the well-developed oblique the rectus is also 

 well developed, but the striations are not distinct. 



The scleral cartilages form one of the striking features of this eye. They are 

 quite variable, forming a more or less complete covering for the eye. In some they 

 are several times as long as the eye and in such cases extend much beyond the 

 eye. In one eye 49 /x. in diameter the length of one of the cartilages reaches 160 fi 

 (fig. 45 a). They have not kept pace in their reduction with the reduction of the 



Fig. 45. (a and b) Two Cross-sections of Eye of Specimen preserved in Alcohol, 38 mm. long. Sec- 

 tions show Variable Extent of Pigment, Choroidal (ch.) Pigment, and Scleral 

 Cartilages. Extent of latter represented by dotted lines in figure a. 



eye in size. As a consequence individual cartilages either extend beyond the eye 

 or are bent at acute angles in their endeavor to apply themselves to the shrunken 

 eye (fig. 46 a, scl.c). These cartilages were mistaken for the suborbital bones by 

 Kohl. There is absolutely no ground for this supposition. The suborbitals are 

 present (fig. 44 a, subo.y and widely separated from these cartilages. Further, the 

 eye muscles are attached to the cartilages and to similar ones in Amblyopsis. 



The presence of these large cartilages is the more remarkable when we con- 

 sider that none are found in Typhlichthys subterraneus, and in the species of 

 Chologaster, which in other respects resemble Typhlichthys in all but the develop- 

 ment of the eye and the color. It is quite evident that Troglichthys and Typh- 

 lichthys are not derived from a common ancestor (except, of course, remotely). 

 Their present superficial resemblances are the result of converging development 

 under similar environments. A species similar to Chologaster agassizii gave rise 

 to Typhlichthys subterraneus. What the ancestry is of Amblyopsis and of Tro- 

 glichthys is not known. The cartilages are bound together by an abundant fibrous 

 connective tissue containing a few corpuscles. (These I have found nowhere as 

 abundantly as represented by Kohl.) 



