xv. 8 LIZARDS AND SNAKES 405 



Although typical lizards preserve a number of primitive reptilian 

 features, the Squamata as a group show several interesting specializa- 

 tions that are absent in Sphenodon, and to which their success may 

 be partly attributed. In the majority of forms, especially in the snakes, 

 the skull is highly kinetic, having a freely movable quadrate, which 

 imparts its motion to the bones of the upper jaw. Paired copulatory 

 organs of a unique type are present in the male. There is a widespread 

 tendency towards limb reduction, which has apparently occurred 



Fig. 232. Jacobson's organ. Diagram of reconstructed L.S. of snout of lizard showing nose 

 and organ of Jacobson, both seen in section from the lateral side. 



ex.n. region of external nostril; in. internal nostril; j.o. (/.) Jacobson's organ lumen ;/.o. (s.e.) 

 Jacobson's organ sensory epithelium; l.d. lachrymal duct, front end, cut; l.n. lining of nose; 

 n.j.o. nerves to Jacobson's organ; o.b. olfactory bulb; o.n. olfactory nerves; /. tongue. 



independently in members of about half the existing families of 

 lizards, and in snakes. 



The paired organs of Jacobson are highly elaborated and of great 

 functional importance. In snakes and in most lizards these organs 

 (Fig. 232) are hollow domed structures above the front of the palate, 

 each opening into the mouth by means of a slender duct. The lachry- 

 mal duct opens into or near the duct of Jacobson's organ, instead of 

 into the nose, suggesting that the secretions of the eye glands may have 

 some special function related to that of the organ. Odorous particles 

 are carried to the ducts of Jacobson's organs, or to the immediate 

 neighbourhood of them, by the tongue tip, which is forked in snakes 

 and many lizards. The lumen of the organ is partly lined by sensory 

 epithelium, supplied by a separate branch of the olfactory nerve. 

 Experiments by Noble and others have shown that the organs assist 

 in such functions as sex recognition and following trails left by prey. 



