160 THE ANATOMY OF VEETEBEATED ANIMALS. 



4. Beak-like compound teeth, attached to the premaxillse 

 and dentary hones of the mandible. 



These are of two kinds. In the Parrotfish [Scarus) the 

 beak is formed by the union of nnmerons separately de- 

 veloped teeth into one mass. But in the Gymnodonts 

 (Tetrodon and Dioclon) the beak is produced by the coales- 

 cence of broad calcified horizontal lamellae thrown off from 

 a subjacent pulj). 



5. In the Carp and its allies the basi- occipital sends down 

 a median process, which expands at the end, and siipports a 

 bi'oad, thick, horny tooth. 



The stomach is usually wide and sac-like, but sometimes 

 (in Scomberesoces, Cyprinoids, and others) is not wider than 

 the intestine. Occasionally, as in Mugil, it acquires thick 

 walls and becomes gizza,rd-like. The commencement of 

 the small intestine is very generally marked by the presence 

 of more or less numerous csecal diverticula, thepyloi-ic cceca. 

 The small intestine has no spiral valve, though the mucous 

 membrane may be raised into large transverse folds. The 

 i"ectum does not terminate in a cloaca, and almost always 

 opens quite separately from the iirinary and genital diicts, 

 and in front of them. 



In many Teleostean fishes an air-bladder underlies the 

 vertebral column, and is connected by an open pneumatic 

 duct with the dorsal wall of the oesophagus, or even with 

 the stomach, as in the Herring. In other Teleostei, the air- 

 bladder occupies the same position, but is closed, the duct 

 Ijy which the air-bladder is primitively connected with the 

 alimentary canal becoming obliterated. In a comparativelj" 

 small number of the Teleostei — the Blennii, the Pleuronec- 

 tid(e or Flatfishes, the Sand-eel (Amviodytes), the Loricarini. 

 and Symbranchii, and some members of other families — 

 there is no air-bladder. In those Teleostei in which it is 

 present, it may be di%'ided into two parts by a constriction ; 

 or it may be prolonged into diverticula ; or retia mirabilia 

 may be developed in its walls. Sometimes the air-bladder 

 is brought into direct relation with the membranous laby- 

 rinth, as in Myripristis and Sp>arus, and the Herring, Shad, 



