4ZO ZOOLOGY FOR MEDICAL STUDENTS chap. 



Reptilia 



1. Rhyncocephalia — Sphenodon, the sole survivor of a compara- 

 tively archaic group of reptiles. It is restricted to certain islands in the 

 Bay of Plenty, New Zealand. 



2. Lacertilia — Lizards, " Bhnd-worms." 



3. Ophidia — Serpents. 



4. Chelonia — Tortoises, Turtles. 



5. Crocodilia — Crocodiles, Alligators. 



For convenience there are grouped along with existing Reptiles a 

 large number of extinct vertebrates. These reptile-like creatures reached 

 their maximum development during the Jurassic period. Their remains 

 show wonderful variety of form, and adaptation to special modes of 

 progression — swimming (Ichthyosaurs, Plesiosaurs), bipedal running and 

 leaping (Dinosaurs), flying (Pterosaurs). Some of them reached a gigantic 

 size, such as Diplodocus which attained to oxer 80 feet in total length. 



Our knowledge of these extinct creatures is practically limited to the 

 bony skeleton. Of all the other important systems of organs we know 

 nothing. We do not even know whether the covering of the body was 

 dry and horny or soft and glandular, nor whether the early stages in 

 development were of the highly specialized type characteristic of the 

 Amniota. Consequently when we speak of them as extinct reptiles we 

 use that term in a somewhat loose sense. 



Restricting the term to existing reptiles we may say that they are the 

 first subdivision of living vertebrates which have proved themselves 

 entirely successful colonists of the dry land. They owe their success 

 primarily (i) to the developmental adaptations already alluded to and 

 (2) to the fact that the epidermis forms on its surface a thick dry horny 

 layer which is highly effective in impeding evaporation from the body- 

 surface. 



The primitive form of the tetrapod body is departed from but little 

 in the case of Sphenodon, the Crocodiles, and most of the Lizards (Fig. 

 177, B, p. 403). On the other hand in the Chelonians the trunk region 

 has become squat and short, while in the Snakes it has become greatly 

 lengthened and the limbs have completely disappeared. It is of great 

 interest that a number of different types of Lizard are at the present 

 time to be, so to speak, caught in the act of evolving in this same direction. 

 Thus in the genus Chalcides, an obvious lizard common round the shores 

 of the Mediterranean, the body is very elongated and the limbs very 

 small, and in particular species the number of toes has been reduced 

 from 5 to 3 while in one species they have disappeared entirely, the 



