COMMON PERCH. 87 



The teeth of the Perch are fine, long, close-set, and they are anchylosed 

 to the bone that supports them. There is much variety in the dentinal struc- 

 tures of the Teleostei. An external cap or tip of enamel is commonly present, 

 but the main body of the tooth is made up of dentine in one of its three chief 

 modifications, tubular, vaso- or osteo-dentine. Anchylosis takes place by bone 

 developed specially in connection with each tooth. The oesophagus passes with- 

 out clear distinction into the stomach, which in the Perch is of the type known as 

 caecal^ i. e. prolonged backwards beyond the pylorus. It is siphonal, i. e. bent on 

 itself, in the Pike. There is a well-marked pylorus, and the first (duodenal) por- 

 tion of the intestine is dilated. It carries three appendices pyloricae, structures 

 which may be absent, e. g. Pike, or present in large numbers, e. g. Salmon, where 

 they have a linear arrangement. They are rarely united in Teleostei into a mass by 

 connective tissue, e. g. Tunny. In the Perch, according to Krukenberg, they secrete 

 merely mucus ; in some other fish they have a pancreatic function. The loop of 

 intestine containing the spleen is short, and there is no external mark of separation 

 between the intestine and rectum. There is however an internal valve, a remnant 

 perhaps of a spiral valve, which is stated to exist only in Chirocentrus (Clupeidae) 

 among Teleostei. The folds of the mucous membrane vary much in character, and 

 villi are rare. Peptic glands are well marked in the stomach of the Perch ; they are 

 sometimes absent. Ciliated epithelium occurs in the pyloric appendages. The 

 epithelium of the intestine is columnar, and contains goblet-cells ; and it throws 

 out pseudopodial processes. The gall-bladder is never absent; its duct in the 

 Perch opens near the base of the pyloric appendage, which crosses the oesophagus. 

 The true pancreas is present in the Perch as a diffuse gland. The lobules are 

 chiefly found along the veins of the pyloric appendages, and the left branch of the 

 portal vein. The air-bladder is simple in shape, occupies the whole of the dorsal 

 portion of the abdominal cavity, is firmly fixed laterally to the body-walls, and its 

 ventral surface is covered by peritoneum. Its walls are thin, and there are many 

 vaso-ganglia or retia mirabilia developed on its ventral surface anteriorly and 

 internally. There is no trace of the air-duct which connects the bladder with the 

 digestive tract in the Teleostei Physostomi. 



The inner surface of the operculum, or more strictly speaking, the posterior 

 edge of the hyomandibular bone, carries a filamentous pseudo-branchia or opercular 

 gill. In the adult Pike and many other Teleostei this structure is hidden under the 

 mucous membrane. It is a functional hyoidean gill in the young, but in the adult 

 it receives arterial blood from the hyoidean artery, and transmits it to the choroid 

 gland of the eye. Each of the first four branchial arches carries gill-filaments 

 arranged in two rows, i. e. they are biserial. These filaments are separated to their 

 base as in all Teleostei. The artery and vein run on the convex side of the arch, 

 the vein at a deeper level than the artery. 



In the heart there is a sinus venosus constituted by the fusion of the right 

 and left ductus Cuvieri. It receives the hepatic veins, and its aperture into the 

 auricle has two thin valve-like folds. The walls of the auricle are thin, and muscles 

 arranged more or less in a network. An anterior and posterior valve guard its 

 entrance into the ventricle. This structure has a thick wall, which in most Teleostei 

 and Ganodei is divisible into two muscular layers, an outer and an inner, separated 

 by a space. This space is lymphatic, and its surfaces are covered by an endothelium. 

 Two valves, a right and a left, guard the passage from the ventricle to the bulbus 



