XI AMPHIBIA 



409 



tube — the trachea. This divides towards its hind end into two branches 

 — the bronchi — terminating in the two lungs in the restricted sense. 



In many of the Amphibia the wall of the trachea with its cartilaginous 

 supports is modified in the region next the glottis to form a larynx or 

 voice-producing organ, the actual sound being piroduced by the vibration 

 of folds of the laryngeal lining (vocal cords) projecting into its cavity. The 

 sound produced is loudest in the males of various species of Frogs and 

 Toads and its volume is in many cases increased by resonating chambers 

 (croaking sacs) which develop as outgrowths of the buccal lining. The 

 sound differs greatly in quality in different species and in many of those 

 inhabiting the tropics consists of highly musical notes and trills. 



As regards the rest of the alimentary canal the only feature that need 

 be mentioned is the appearance of an important new organ — the allantois 

 — a thin-walled pocket projecting from the ventral wall of the alimentary 

 canal close to its hinder end and functioning as a urinary bladder in 

 which the secretion of the kidneys accumulates. As will be seen later 

 the precociously developed and enlarged allantois plays a very important 

 part in the embryos of vertebrates above the Amphibia. 



The coelomic arrangements show, as compared with those of fishes, 

 an advance which is maintained throughout the higher vertebrates, in 

 that the pericardiac cavity has become as it were telescoped back into 

 the peritoneal cavity, while its bounding wall has become thin and mem- 

 branous. Consequently the heart of an amphibian or any higher verte- 

 brate appears to lie in the general peritoneal cavity, ensheathed in a thin 

 smooth membrane — this latter being in reality the wall separating the 

 pericardiac from the peritoneal cavity. 



The kidney during larval life is a pronephros the number of whose 

 functional tubules varies from about a dozen in the Apoda down to two 

 in Urodeles. In the adult this is replaced by the opisthonephros. In 

 the common Frog (Rand) a remarkable peculiarity is found in the adult 

 inasmuch as the ciliated peritoneal funnels which stud the ventral 

 surface of the kidney lose their connexion with the Malpighian bodies 

 and come to open into the venous spaces (belonging to the posterior 

 cardinal vein) lying between the tubules. Here we have an arrangement 

 for returning coelomic fluid from the peritoneal cavity back into the blood 

 — a phenomenon which we find again in the Mammalia though in them 

 it is carried out in a different fashion by means of the lymphatic system. 



The eggs are shed from the ovaries into the peritoneal cavity and pass 

 thence by typical MuUerian ducts^-usually long and convoluted and pro- 

 vided with glandular hning which secretes protective envelopes round 

 the eggs in their passage. 



