VERTEBRATA. 



349 



attaches to its significance. The anterior limit of the oral part of the 

 mesenteron is lost except in Cyclostomi, where . it is indicated by the 

 velum. The pharynx of Vertebrata represents a portion and a portion 

 only of the primitive respiratory or branchial section of the fore-gut. Its 

 walls are perforated by a series of branchial or visceral clefts. The greatest 

 number of these known in an adult form is eight, of which the first is the 

 spiracle lying between Meckel's arch and the hyoid (cf. note, p. 338), the 

 position in which the Eustachian tube develops in higher Vertebrata. 

 The remainder are branchial clefts, or in Mammalia and Sauropsida where 

 branchiae do not exist even in the embryo, visceral clefts, limited to four 

 in number. The clefts are formed by hypoblastic outgrowths from the 

 throat reaching to the epiblast which eventually thins away over them, thus 

 leaving an aperture. The septum or wall between each pair of clefts is 

 supported by a branchial arch, and contains an aortic, i.e. a vascular arch. 

 The anterior and posterior walls of the pouches bear vascular processes, 

 branchiae or internal gills in Pisces. External gills, such as are found in 

 Amphibia, permanently or temporarily, are outgrowths covered with epi- 

 dermis near the dorsal ends of the branchial arches. There is reason to 

 believe that the branchial clefts were very numerous in the ancestral Ver- 

 tebrata, and that the true digestive portion of the alimentary canal com- 

 menced, as in Amphioxus, with the region to which the liver is attached. 

 The oesophagus and stomach are separated by a cardiac constriction little 

 marked in most Ichthyopsida. The stomach and mid-gut are marked off by 

 a similar constriction known as the pylorus or pyloric valve, due to great 

 development of the circular muscular coat of the alimentary canal. The 

 rest of the tract varies much in calibre, length, and consequently in the 

 degree to which it is coiled. In Mammalia the terminal section of the 

 colon is straight, and is hence termed rectum. The intestine terminates in 

 the embryo in a cloaca, common to it and the urogenital ducts, a condition 

 which persists in some Pisces, all Amphibia, and Sauropsida and the Proto- 

 theria among Mammals. But the primitive condition is lost in other cases, 

 and the rectum opens separately from the urogenital ducts, as in Holocephali, 

 Teleostei among Pisces, Eu- and Meta-theria among Mammalia ; and occa- 

 sionally the urinary ducts apart from the genital (Holocephali and some 

 Teleostei, the female Rat and a few other Mammalia}. The walls of the 

 alimentary canal consist typically from without inwards of a serous coat ; of 

 a longitudinal and a circular coat of non-striated muscle-cells ; of a sub- 

 mucous connective tissue ; and a mucous coat, the two latter often thrown 

 into variously disposed folds in the mid-gut. The sub-mucous coat con- 

 tains blood-vessels, lymphatic tissue and vessels. The mucous coat is a 

 single layer of epithelial (hypoblast) cells, which form tubular glands in the 

 stomach of all Vertebrata, in the intestine of Amphibia and higher classes. 

 Two glands, a liver and a pancreas, are connected with the commence- 



