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 Atrdozoa. 
As development proceeds, however, it becomes completely separated 
from the pharynx and gives rise to the ¢hyrodd gland. The thymzts 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 stomodeum ; this gives off a dorsal diver- 
ticulum called the yfophysis which may be homologous with the 
subneural gland of the Zznzcata. Its distal end becomes detached and, 
coming into close relationship with the infundibulum of the brain, forms 
the petuctary 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 
Jungs, in terrestrial forms, arise as a single ventral diverti- 
culum of the cesophagus 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; 
