PHYLUM CHORDATA 



361 



and the same holds good of the hinder part of the elongated nasal 

 chamber of the Crocodilia. 



Jacobson's organs (Fig. 972) are present in Lizards and 

 Snakes, absent in Chelonia and Crocodilia in the adult condition. 



The eyes are relatively large, with a cartilaginous sclerotic in 

 which a ring of bony plates (Fig. 973) is developed in some cases. 

 The muscular fibres of the iris are striated. A pecten is present in 

 most. Most Reptiles have both upper and lower eyelids and nicti- 

 tating membrane. The greater number of the Geckos and all 



^■^"i'\h 



Fig. 1005.— Section of the pineal eye o£ Sphenodon punctatum. g, blood-vessels ,■ h, cavity 

 of the eye filled with fluid ; A-, capsule of connective-tissue ; L lens ; m. molecular layer of 

 the retina ; r. retina ; st. stalk of the pineal eye ; a-, cells in the stalk. (From Wiedersheim's 

 Comparative Anatomy, after Baldwin Spencer.) 



the Snakes constitute exceptions, movable eyelids being absent in 

 both of these groups ; in the former the integument passes uninter- 

 ruptedly over the cornea with a transparent spot for the admission 

 of the light ; in the Snakes there is a similar modification, but the 

 study of development shows that the transparent area is derived 

 from the nictitatinsr membrane ■which becomes drawn over the 

 cornea and permanently fixed. In the Ghamseleons there 

 single circular eyelid with a central aperture. 



IS a 



