ALIMENTARY CANAL. 197 



circular canals open, and a lower the saccule {s), which possesses 

 a small posteriorly directed process {lagena) representing the 

 cochlea. The otoliths vary much in form in different fishes. 

 They occur in the utricle, saccule and lagena. That in the 

 saccule (the sagitta) is generally the largest and of a crystalline 

 structure. There is another small one {asteriscus) in the lagena, 

 and a third in the utricle close to the ampullae of the anterior 

 and horizontal canals (o). The ductus endolymphatic us is 

 present as a process of the saccule, but does not open externally. 



The lateral line has already been described (p. 187). Its 

 sense organs are probably innervated by branches of the facial 

 and lateral line branch of the vagus as in other fishes, but this 

 has not been shown in all cases. 



The alimentary canal is distinguished by the very general 

 presence of an air bladder, which must be regarded as an appen- 

 dage of the oesophagus, though it is often in the adult separate 

 from this ; by the presence of more or less numerous appendages 

 — the pyloric caeca, opening into the first part of the intestine ; 

 by the inconspicuous character of the pancreas, which in some 

 cases is even said to be absent ; by the absence of a spiral valve 

 in the intestine ; and by the absence of a cloaca common to the 

 alimentary canal and urinogenital organs. 



The teeth * are usually well-developed, but in some cases are al- 

 together absent (some Lophobranchii , Coregonus). They may be 

 borne by the maxillary, premaxillary, palatine, vomer, dentary 

 bones, by the glosso-hyal and by the branchial arches, and rarely 

 by the pterygoid and parasphenoid. The maxilla is usually with- 

 out teeth, and does not always form part of the edge of the mouth. 

 The teeth are generally ankylosed to the subjacent bony 

 structures, but in some cases there is a ligamentous connection 

 of such a kind that they can be bent inwards when food is being 

 swallowed, but not in the reverse direction (some Gadidae, 

 Lophius, Esox). In a few cases they are implanted in sockets 

 {Sphyraena, etc.). As a rule teeth continue to be developed 

 throughout life from new germs (not from pre-existing germs), 

 placed behind the functional teeth. These come into function 

 and position as the old teeth are worn down and cast off. When 

 the teeth are implanted in sockets, the new tooth, though 



* R. Owen, Odontography, London, 18-40-i5. C. S. Touies, Dental 

 Anatomy, London, LS98. 



