XIII 



PHYLUM CHORPATA 



301 



and tlie same holds good of the liinder part of the elongated uasal 

 chamber of the Crocodilia. 



Jacobsou'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 

 wliich a ring of bony plates (Fig, S)73) is developed in some cases. 

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

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

 tating membrane. The greater number of the (leckos and all 



Fio. 1005.— Section of the pineal eye of Sphenodon punctatum. <7, blood-vessels ; /(, cavity 

 of the eye filled with fluid ; /•, capsule of connective-tissue ; !. lens ; ra. molecular laj'er of 

 the retina ; r. retina ; st. stalk of the i>ineal eye ; .c, cells in the stalk. (From Wieder.sheim's 

 Comparative Anatomi/, after Baldwin Spencer.) 



the Snakes constitute exceptions, movable eyelids being absent in 

 botli 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 nictitating membrane which becomes drawn over the 

 cornea and permanently fixed. In the Chama4eons there is a 

 single circular eyelid with a central aperture. 



