XII REPTILIA 423 



out merely into shallow sacculations. On the other hand in the larger 

 reptileSj such as the Crocodiles and the large Turtles, the outgrowths of 

 the lung lining are greatly increased in length and their walls project 

 into secondary sacculations, the whole forming a thick respiratory sponge- 

 work surrounding a comparatively narrow tubular central cavity or 

 intra-pulmonary bronchus. 



A point of general interest is that tetrapods in which the body has 

 become much elongated in form commonly show the same asymmetry of 

 the two lungs as occurs in the Crossopterygians or the young Lung-fish 

 — the left lung being reduced in size. This is well shown by the various 

 reptiles that have assumed a snake-like form. The asymmetry may be 

 comparatively slight as in the Boa, or the left lung may disappear entirely 

 as in the Adder. In the latter, as in various other snakes, a further 

 modification has taken place — in that the hinder part of the right lung has 

 become smooth and thin-walled and has lost its respiratory function. 



In the Chameleons also parts of the lung-wall have lost their respiratory 

 function, but here these parts of the lung take the form of pocket-like 

 outgrowths which the Chameleon is able to blow up with air when alarmed. 

 Such outgrowths of the lung are of much morphological interest for they 

 foreshadow remarkable developments characteristic of the lung of Birds. 



In Fish and Amphibians the lungs are filled by air being forced into 

 them from the mouth. In Reptiles on the other hand we find for the 

 first time " costal respiration " taking place, air being drawn into the 

 lungs by movements of the ribs. In the Chelonians where this is im- 

 possible, the ribs being fixed in position, air is drawn in by the flattening 

 of a curved sheet of muscle forming a floor underneath the lungs. 



The allantoic bladder, in spite of its great size in the embryo, dis- 

 appears as a rule during the development of the reptile, although in 

 Chelonians and Lizards part of it persists as a functional urinary bladder 

 in the adult. 



As regards the coelomic organs the chief point to notice is that the 

 nephridial organs have assumed the condition characteristic of the 

 Amniota in general. The pronephros appears merely in the embryo as 

 an inconspicuous vestige. The opisthonephros is differentiated into 

 mesonephros and metanephros. The mesonephros functions as kidney 

 during the developmental stages but in the adult its place as such is 

 taken by the metanephros, and what remains of the mesonephros fulfils 

 a purely sexual function. It lies in contact with the testis, forming the 

 epididymis, and its tubules serve to convey the spermatozoa into the 

 Wolffian duct or vas deferens. 



In the heart of the reptiles the most interesting advance has to do 



