TELEOSTEAN FISHES 64I 



bent upwards as usual, the subsequent development of rays 

 produces an apparent symmetry. The scales are in most 

 cases relatively soft. The roof of the fore-brain is without 

 nervous matter. The optic nerves are remarkable, because 

 they cross one another without interlacing (decussate). 

 The partitions between the gill-clefts disappear ; so, in- 

 stead of the pouches seen in Elasmobranchs, there is, on 

 each side, one branchial chamber, covered over by an 

 opercular fold. Into this chamber the comb-like gills, 

 borne by the branchial arches, project freely. There is 

 usually a rudimentary gill or pseudobranch associated with 

 the hyoid. There is no spiracle. In most types there 

 is a swim-bladder, which grows out from the gullet. 

 The duct of the swim-bladder may remain open (Physo- 

 stomatous), as in herring, salmon, and carp ; or it may be 

 closed (Physoclystous), as in perch and cod. There is no 

 spiral valve in the intestine, and the food canal ends in 

 front of, and separate from, the genital and urinary aper- 

 tures or aperture. The base of the ventral aorta is swollen 

 into a non-contractile bulbus arteriosus, but there is no 

 conus, unless very exceptionally, as in Butirinus. A re- 

 markable peculiarity is that the gonads are usually con- 

 tinuous with their ducts. The ova are numerous, usually 

 small and fertilised in the water. The segmentation 

 is meroblastic, and there is usually a distinct larval 

 stage. 



The Teleosts include the great majority of living fishes, which are 

 classified in thirteen sub -orders and numerous families, e.g. Clupeidas 

 (herrings) ; Salmonidae (salmon, trout) ; Cyprinidae (carps) ; Murasnida? 

 (eels) ; Esocidae (pike) ; GasterosteidaB (sticklebacks) ; Syngnathidce 

 (pipe-fish and sea-horses) ; Gadidae (cod-fishes) ; Percidae (perch) ; 

 Scombridse (mackerels) ; Pleuronectidae (flat-fishes) ; Cottidae (bull- 

 heads) ; Triglidae (gurnards) ; Lophiidas (anglers) ; Tetrodontidce 

 (globe-fishes). 



Sub-Class III. Dipnoi. Lung- or Mud-Fishes 



This interesting group of fishes, whose name means 

 double breathers, is now represented by three genera, 

 surviving remnants of a very old stock : Ceratodus, from 

 the Burnett and Mary rivers of Queensland ; Protopterus, 

 from various rivers and marshes of West Africa, e.g. 



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