THE ALIMENTARY CANAL. 391 



persist as practically functionless residual structures. In 

 some cases they do not even open. The clefts are sup- 

 ported by the branchial arches described in connection with 

 the skull, and are supplied by blood-vessels. 



With the most anterior part of the alimentary canal, two 

 strange structures are associated — (a) the thyroid arising as 

 a ventral constriction, perhaps comparable to the endostyle 

 of Tunicates, perhaps substituted for a pair of gill-clefts ; 

 (p) the thymus arising in connection with the posterior gill- or 

 visceral-clefts. Very little is known in regard to the function 

 of these bodies, but the thyroid is connected with the blood 

 supply to the head, while the thymus usually atrophies in 

 the adolescence of higher Vertebrates. 



The pharynx leads into the gullet or oesophagus, which is 

 a conducting tube, and this into the digestive stomach, which 

 is followed by the digestive and absorptive and conducting 

 intestine, ending in the rectum and anus. 



From the oesophagus, the air- or swim-bladder of most 

 Fishes, and the lungs of higher Vertebrates, grow out. The 

 air-bladder lies dorsally and is almost always single ; the 

 lungs lie ventrally and are double, though connected with 

 the gullet by a single tube. It is not certain that these out- 

 growths are homologous, though the air-bladder of Dipnoi 

 acquires the functions of a lung. 



The beginning of the intestine gives origin to the liver 

 which regulates the composition of the blood and secretes bile, 

 and to the pancreas which secretes digestive juice. 



From the hindmost region of the gut, the allantois grows 

 out in all animals from Amphibians onwards. In Amphi- 

 bians this is simply a bladder ; in the higher Vertebrates 

 it is a birthrobe, respiratory or nutritive, or both. 



In a few cases, the end of the alimentary canal is simply 

 the opening or blastopore of the original gastrula cavity or 

 archenteron. This is true in the lamprey, and in at least 

 several Amphibians such as frog and newt. In other cases, 

 an ectoderraic invagination or proctodseum meets the closed 

 archenteron. 



Behind the anus, there is in some embryos what has been 

 called the "post-anal gut." It seems very likely that this is 

 connected with the neurenteric canal, which unites the 

 dorsal nervous tube with the ventral alimentary tube. 



