510 FISHES. 



epochs that they assume among fishes that overwhelming 

 preponderance in numbers which they possess at the present 

 day. The physostomous type of Teleostean is the most 

 ancient, and probably stands in a continuous genetic line 

 with the Lepidosteoid Ganoids. 



The skeleton is well ossified, with numerous investing 

 bones on the skull, others in the operculum, and on the 

 shoulder girdle. The tail is sometimes quite symmetrical 

 or diphycercal, but in most cases it is heterocercal at first, 

 and acquires a secondary symmetry termed homocercal, for 

 while the end of the notochord in the young forms is bent 

 upwards as usual, the subsequent development of rays pro- 

 duces an apparent symmetry. The scales are in most cases 

 relatively soft. As in Ganoids, the roof of the fore-brain is 

 without nervous matter. The optic nerves are remarkable, 

 because they cross one another without fusing (decussate). 

 As in Ganoids, the partitions between the gill clefts dis- 

 appear, so instead of the pouches seen in Elasmobranchs, 

 there is, on each side, one branchial chamber, covered over 

 by an opercular fold. Into this chamber the gill lamellae 

 borne by the branchial arches project freely. In most, a 

 swim bladder is developed from the dorsal side of the gullet. 

 There is no spiral valve in the intestine, and the food canal 

 ends in front of and separate from the genital and urinary 

 apertures 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. 

 According to some authorities, the archinephric duct is 

 unsplit, and there is no Miillerian duct ; according to 

 Jungersen, the oviduct is a true Miillerian duct. The 

 pronephros degenerates ; the ova are numerous, and are 

 fertilised in the water. 



Classification of Tekostei. (After Giinther.) 



f Acanthopteri. Example Perch/ 

 Dorsal, anal, and pelvic J 



fins in part spiny. J Pharyngognathi. Example 



Wrass. 

 fAnacanthini ; the pelvic fins are 



The dorsal, anal, and situated far forward. Examples 

 pelvic fins without^ -Cod, Flounder. 



Physoclisti, duct 

 of swim bladder is 

 closed. 



pelvic nns witnoutx 



I Physostomi ; duct of swim bladder remains open. Ex- 

 V. amples Herring, Salmon, Carp, Eel. 



