XIII 



PHYLUM CHORDATA 



335 



Sensory Organs. In most Lacertilia, but not in the Ophidia, 

 the nasal cavity consists of two parts an outer or vestibule and 

 an inner or olfactory chamber the latter having the sense-cells in 

 its walls, and containing a turbinal bone. In the Turtles each 

 nasal chamber is divided into two passages, an upper and a lower, 

 .and the same holds good of the 

 hinder part of the elongated nasal 

 chamber of the Crocodilia. 



Jacobson's organs (Fig. 917) 

 are present in Lizards and 

 Snakes, absent in Chelonia and 

 Crocodilia in the adult condi- 

 tion. 



The eyes are relatively large, 

 with a cartilaginous sclerotic in 

 which a ring of bony plates (Fig. 

 918) 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 

 the Snakes constitute exceptions, 

 movable eyelids being absent 

 in both these groups ; in the 

 former the integument passes un- 

 interruptedly over the cornea 

 with a transparent spot for the 

 admission of the light; in the 

 Snakes there is a similar modifi- 

 cation, but the study of the de- 

 velopment shows that the trans- 

 parent area is derived from the 

 nictitating membrane which .be- 

 comes drawn over the cornea 

 and permanently fixed. In the 

 Chameleons there is a single 

 circular eyelid with a central 

 aperture. 



The middle ear is absent 



NH- 



Med, 



Xf 



MM 



11 of Alligi 



tory bulb ; 



HH, cerebellum ; Med, spinal cord ; M, a, 

 optic lobes ; NH, medulla oblongata ; 

 VH, cerebral hemispheres ; I XI, cranial 

 nerves ; 1,2, first and second spinal 

 nerves. (From Wiedersheim.) 



FIG. 947. Brain of Alligator, from above 

 B. ol. olfactory bulb ; G, p, epiphysis ; 

 HH, cerebellum ; Med, spinal cord ; M, H, 



in 



the Snakes, though a columella auris is present, embedded in 

 muscular and fibrous tissue. 



Developed in close relation to the epiphysis there is in many 

 Lizards (Lacerta, Varanus, Anguis, Grammatophora and others) and 

 in Hatteria, a remarkable eye-like organ the pineal eye (Fig. 948), 

 which is situated in the parietal foramen of the cranial roof 



