334 CHORDATA. 
portion is sometimes distinguished as the rectum. The lumen is simple 
and has no spiral valve. 
From the dorsal wall of the cesophagus there is produced a solid~ 
cord of connective tissue, which is connected at its distal end with a 
large and spacious az7-bladder lying immediately above the abdominal 
cavity. It is filled with gases and its walls have a dense vascular supply. 
This air-bladder is used as a hydrostatic apparatus and is not found in 
demersal fish (those habitually frequenting the bottom). In many, e¢., 
the herring, the connecting cord is a duct putting its cavity into communi- 
cation with that of the cesophagus. It always arises in the young as a 
diverticulum of the alimentary canal. 
In the mesentery above the ileum is a small red sp/eex. Dorsally 
to the abdominal cavity and to the air-bladder lies a pair of elongated 
kidneys of a dark-red colour. They are thin in the 
Excretory. region above the air-bladder, but swell out anteriorly 
immediately behind the head into large bulbous organs, 
and also posteriorly where they give off an unpaired zreter passing down 
to the zrzzary aperture. It swells into a small urinary bladder near 
the aperture. These kidneys are said to be mesonephric in origin. 
_ The heart is smaller in proportion than in the skate. It has two 
chambers, an azricle and a ventricle. The former is fed from a thin- 
BI walled szzzas venosus, and the latter leads forwards as the 
000- branchial artery. There is no valvul teri 
Vascular, 27@”chtal artery. ere is no valvular conus arteriosus, 
as in the skate, its vestige being seen in a single pair of 
valves ; there is a swollen base to the branchial artery sometimes dis- 
tinguished as the dzdbus arteriosus. ‘The branchial artery gives off four 
paired afferent branchials to the gills which give fine branches to the 
gill-filaments. The blood after eration is collected by four pairs of 
efferent branchzals in the roof of the mouth, which are difficult to follow. 
The efferent branchials of each side unite to form a vessel often termed 
the epibranchial artery. Anteriorly each epibranchial is continued 
forwards to meet its fellow across the base of the skull, completing 
the so-called cephalic cercle. Each gives off a carotid to the head. 
Posteriorly each epibranchial converges towards the middle line, and 
gives off a subclavian artery to the Zectoral fin. They then unite to 
form the dorsal aorta, which runs backwards immediately below the 
vertebral column. It can be seen between the kidneys on removal of 
the air-bladder. Posterior to the abdominal cavity it divides into the 
caudal artery, supplying the tail-muscles and the veszczar artery to the 
urinary bladder and anal fin. The dorsal aorta gives off numerous 
renals to the kidneys throughout its course. From the right epi- 
branchial anterior to the origin of the subclavian there arises a pair of 
median w7sceral arteries. The anterior of these supplies the pyloric 
ceca, and the posterior, sometimes known as the ce/éaco-mesenteric, gives 
branches to the stomach, intestine, air-bladder, spleen and gonads. 
The venous system is difficult to follow except in injected specimens. 
It consists of paired Zrecavals leading out from the sinus venosus, 
which give off jugudars forwards and cardinals backwards. There are no 
lateral veins. The cardinals run in the kidneys and receive numerous 
renal veins. The caudal vein is large and runs forwards immediately 
below the caudal artery. At the level of the posterior portion of the 
