PISCES. 



421 



or by special ' bone of attachment/ the most common mode. The forma- 

 tion of new teeth is in most instances perpetual, and in Teleostei the new 

 enamel germs are derived from the rete mucosum, and not, as usual, from 

 pre-existing germs. Vertical replacement is found in the pharyngeal 

 teeth of Pharyngognathi. 



Fish have no oral salivary glands. The tongue is a thickening of the 

 mucous membrane covering the basihyal, with free edges and apex in 

 Elasmobranchii and Polypterus. In the latter it contains muscular tissue, 

 and special lingual muscles are said to be present in Dipnoi. The mouth 

 is a curved or straight transverse aperture, ventral in Elasmobranchii, 

 terminal in other Fish. The pharynx is perforated laterally by the gill- 

 slits. The digestive tract takes a nearly straight course in Holocephali, a 

 few Teleostei) and Dipnoi. The oesophagus is scarcely marked off from the 

 stomach. It has sometimes pointed processes directed towards the stomach 

 (some Elasmobranchii and Teleostei] ; large and branched in the basking 

 Shark (Selache). Ciliated epithelium occurs in it in Elasmobranchii and 

 Ganoidei. The stomach is a dilatation followed by a pyloric constriction in 

 some Teleostei, e. g. Gobius, Protopterus ; a siphonal bend, or a caecal 

 pouch with a lateral pylorus in Amia, Polypterus, and very many Teleostei. 

 In some of the latter the gastric glands have an epithelium unlike that of 

 the stomach. The intestine is usually dilated at the pylorus, and in 

 Elasmobranchii the dilatation is known as Bursa Entiana. It gives off in 

 this region, in Ganoidei and most Teleostei (except e. g. Siluroidei, Esox, 

 Plectognathi) caecal processes or appendices pyloricae, variable in number ; 

 arranged in a whorl or a linear series (Salmonidae) ; opening each by a 

 separate aperture or united into bundles with a common duct ; free or 

 bound together by connective tissue into a solid mass in Acipenser and a 

 few Teleostei, e. g. Tunny (Thynnus]. Their epithelium may be ciliated 

 (cf. p. 87). The remainder of the intestine is often straight, but in Teleostei 

 may be somewhat convoluted. It contains in Elasmobranchii, Holocephali, 

 Ganoidei, and Dipnoi a spiral ridge or valve, very well-developed in the 

 two orders first named ; but reduced in chondrostean Ganoidei, Lepidosteus, 

 and Amia. In some Elasmobranchii, e.g. Zygaena, Carcharias, it has the 

 form of a lamella rolled upon itself. A rudiment of it is said to exist in 

 the Clupeid Chirocentrus (Teleostei). The rectum is clearly marked off 

 from the intestine by being dilated in Ganoidei. There are no intestinal 

 glands, but goblet mucous cells occur in the epithelium. The rectum is 

 lined by the same epithelium as the intestine, except in Elasmobranchii, 

 where it is squamous. Ciliated epithelium is found on the edge of the 

 spiral valve in Elasmobranchii or here and there in some Teleostei, e.g. John 

 Dory (Zeus faber). Peculiarities are the presence of a rectal gland in 

 Elasmobranchii ; striation of the muscular coats of the digestive tract in 

 the Tench (Tinea} ; the presence of capillaries between the epithelium cells 



