VERTEBRATA. 425 



lining (or peritoneum) which in most cases forms a mesentery 

 dorsally where the splanchnic layer joins the somatic. The 

 perivisceral cavity becomes divided into pericardial and 

 abdominal cavities, and in mammals there is a further 

 separation of two pleural cavities. 



Alimentary System. — The most outstanding feature 

 of the vertebrate alimentary system is the presence of paired 

 pharyngeal clefts which arise as hypoblastic pockets, growing 

 out into contact with the epiblast and then opening to the 

 exterior. In fishes these pharyngeal clefts function as gill- 

 slits, the hypoblastic epithelium growing out into gill-fila- 

 ments. The first pharyngeal cleft appears in the skate to 

 have already lost its branchial function, and serves only as 

 a spiracle or aperture for introduction of water. In many 

 fishes the mouth is used for this purpose and the first cleft 

 is then given up. 



In the terrestrial Vertebrata the first pharyngeal cleft 

 persists as the Eustachian canal and middle ear whilst all 

 the others atrophy. They are found more or less distinctly 

 in the embryo, but are merely vestigial organs. 



In the mid-ventral line of the pharynx in vertebrate embryos there 

 arises a groove having the same relationships as the endostyle of Atriozoa. 

 As development proceeds, however, it becomes completely separated 

 from the pharynx and gives rise to the thyroid gland. The thymus also 

 appears to arise by several rudiments in connection with the gill-slits. 

 The extreme anterior part of the alimentary canal is formed by an 

 epiblastic ingrowth called the stomodceum ; this gives off a dorsal diver- 

 ticukmi called the hypophysis which may be homologous with the 

 subneural gland of the Tunicata. Its distal end becomes detached and, 

 coming into close relationship with the infundibulum of the brain, forms 

 the pituitary body. 



The alimentary canal is in its earliest condition a simple 

 tube, but certain parts, such as the pharynx and stomach, 

 develop by rapid growth into large sac-like swellings. The 

 lungs ^vn terrestrial forms, arise as a single ventral diverti- 

 culum of the oesophagus which forks into two, and each 

 becomes distended into a sac. The sac becomes the lung 

 and the connecting stalk persists as the trachea and bronchi. 

 Behind the stomach the intestine buds out a ventral diverti- 

 culum which forms the liver, its stalk becoming the bile-duct; 



