ADNEXA IN AMPHIBIA 419 



Permanently aquatic salamanders and frogs, and the larvae of all 

 amphibians, have no lids or special ocular glands. In adult land sala- 

 manders a distinct, thick skin-fold forms an upper lid and a thinner, 

 mobile lower lid is present. Its transparent border moves upward to close 

 the eye, and the lid is lubricated by a row of compound glands in its 

 lining. These may be best developed nasally and temporally; and the 

 intervening glands may even be lacking, so that two masses of glands 

 are isolated — the forerunners of the serous lacrimal gland (temporally) 

 and the sebaceous Harder's gland (nasally) . 



In the anurans the transparent portion of the lower lid has been elab- 

 orated and is retractible within the remainder to form a Z-like fold 

 (Fig. 106, p. 266) — often loosely termed a 'nictitating membrane' though 

 it has no phylogenetic connection with the true nictitans of higher forms, 

 or with that of the requin sharks. The thickened rim of the lower lid 

 continues completely around the posterior of the eyeball as a cord, which 

 passes through the retractor bulbi (see Fig. 143a, p. 421). When this 

 muscle contracts, the eyeball is pulled into the head and forms a bulge in 

 the roof of the mouth, which is of considerable aid in the swallowing of 

 food. The resultant tug on the cord pulls out the fold of the lower lid 

 and slips the latter up over the cornea to meet the motionless upper lid. 

 A broad hammock-like muscle behind the eye, the levator bulbi, raises 

 the globe once more to its normal elevated position and the lower lid 

 automatically slips down into its folded attitude. The eye can close with- 

 out complete retraction; but the retractile closure, and the muscles con- 

 cerned, are important for protection against mechanical pressure and 

 blows. The frog having no flexible neck, the eye must be able to dodge, 

 since the head as a whole cannot! The single large gland present is con- 

 sidered to be the Harderian, and spreads into the orbit at metamor- 

 phosis to take a position among the muscles behind the eyeball. There 

 are apertures, at the middle and at the nasal end of the lower lid, which 

 communicate with a nasolacrimal or tear duct. This tube lies chiefly in 

 the skin and runs horizontally to the small nasal cavity. 



The permanently aquatic anurans (see p. 407), as might be expected, 

 have secondarily lost the lids, and probably most or all of the special 

 muscles and glands developed by terrestrial amphibians. The adult eye 

 still peers through the primary spectacle of the tadpole. 

 The Third Lid and the Fate of the Retractor — In the Sauropsida, 

 the lower lid still characteristically does all the work of closing the eye; 

 but being thicker than in the frog, and moreover rendered opaque as a 



