602 AMPHIBIANS 



The cornea of this form is also abnormal in that it contains blood ves- 

 sels (but the ca. 9mm. eyeball could not properly be called degenerate) . 

 The closely related Cryptobranchus alleghaniensis also has an overgrown 

 scleral cartilage, equal in thickness to the radius of the lens. TTie con- 

 nective-tissue capsule of the minute and vestigial eye of the only Eu- 

 ropean cave salamander, Proteus, contains only bits of cartilage; and 

 the cartilage in the sole American proteid, Necturus, is also discontin- 

 uous. Cartilage is sometimes present in the permanently larval American 

 cave salamander Typhlomolge, but occurs in its relative, Typhlotriton, 

 only prior to the metamorphosis which this cave salamander, alone of all 

 such, experiences. In general, then, it may be said that whereas the 

 anurans lack scleral cartilage as larvae and possess it as adults, in the 

 urodeles this situation is reversed. It would be interesting to know 

 whether the tadpole of Pseudacris has scleral cartilage (see p. 595). 



The chorioid is relatively thicker than in anurans, but more loosely 

 organized, with pigmented connective-tissue membranes running in quite 

 helter-skelter fashion. The circulation of the chorioid and iris is much 

 as in Rana, but the details have not been as well worked out for any 

 urodele. The ciliary body is triangular in meridional section and much 

 smaller than in anurans. There are no folds on the ciliary body or the 

 iris, excepting the single mid-ventral 'ciliary process' into which the 

 accommodatory muscle inserts. The iridic portion of this process is essen- 

 tially a seam formed by the closure of the embryonic fissure of the optic 

 cup — such a seam is quite generally present in lower vertebrates, run- 

 ning all the way to the pupil margin.* Urodeles have no canal (s) of 

 Schlemm; but crescentic ciliary muscles are present dorsally and ven- 

 trally in some forms (though not, apparently, in Ambystoma) . 



There are no pupillary nodules, but otherwise the structure of the iris 

 is like that of the frog's. There is only a single, ventral, protractor 

 lentis muscle. This appears to be strictly comparable with its anuran 

 counterpart — and it will be recalled that an anuran can lack the dorsal 

 muscle. The lens is relatively larger and more nearly spherical (especially 

 in larvae and aquatic adults) than in the Anura (e.g., Necturus — equa- 

 torial diameter only 1.05 >; axial). It is most strongly supported by the 

 mid-dorsal fibers of the anuran-like zonule (cf. fishes), less well by 

 the mid-ventral ones, and depends least upon those in other sectors. 



*It will be recalled that in mammals (see pp. 115-6) the blind part of the retina is not a 

 portion, but rather an outgrowth, of the optic cup; hence, it never normally contains a por- 

 tion of the embryonic fissure, which has healed before the commencement of the outgrowth. 



