334 CHORD AT A. 



portion is sometimes distinguished as the rectum. The lumen is simple 

 and has no spiral valve. 



From the dorsal wall of the oesophagus there is produced a solid 

 cord of connective tissue, which is connected at its distal end with a 

 large and spacious aij'-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.g.^ 

 the herring, the connecting cord is a duct putting its cavity into communi- 

 cation with that of the oesophagus. It always arises in the young as a 

 diverticulum of the alimentary canal. 



In the mesentery above the ileum is a small red spleen. 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 tireter passing down 



to the urinary 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 auricle and a ventricle. The former is fed from a thin- 

 Rl od walled sinus ve7iosus^ and the latter leads forwards as the 



Vascular branchial artery. There 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 bulbus arteriosus. The branchial artery gives off four 

 paired afferent branchials to the gills which give fine branches to the 

 gill-filaments. The blood after aeration is collected by four pairs of 

 efferent branchials 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 circle. Each gives off a carotid to the head. 

 Posteriorly each epibranchial converges towards the middle line, and 

 gives off a subclavian artery to the pectoral 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 vesicular 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 visceral arte^Hes. The anterior of these supplies the pyloric 

 ccBca, and the posterior, sometimes known as the ccEliaco-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 precavals leading out from the sinus venosus, 

 which give oi^ jugulars forwards and cardinals backwards. There are no 

 lateral veins. The cardinals run in the kidneys and receive numerous 

 I'enal veins. The caudal vein is large and runs forwards immediately 

 below the caudal artery. At the level of the posterior portion of the 



