TMLEOSTEI. 431. 



the young forms is bent upwards as usual, the subsequent 

 development of rays produces 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 ; in other words they are decussate. As in Ganoids, 

 the partitions between the gill-clefts disappear, so instead of 

 the pouches seen in Elasmobranchs, there is, on each side, 

 one branchial cham.ber, covered over by an opercular fold. 

 Into this chamber the gill-filaments 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 exception- 

 ally, 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 Teleostei. (After Gtinther.) 



Before mentioning the principal sub-orders of Teleostei, it is con- 

 venient to distinguish two sets of aberrant forms : — 



(a) The sea-horses, such as Hippocampus and Phyllopteryx, and the 

 pipe-fishes, such as Syngnathtis, are distinguished as Lopho- 

 branchii. The gills, instead of being rows of filaments, are 

 tufts of rounded lobes ; the gill-cbver is a simple plate leaving 

 a small aperture ; the skin is more or less protected by large 

 dermal plates ; the toothless mouth is at the end of a prolonged 

 snout. 

 {b) The globe-fishes, such as Tetrodon and Diodon, the trunk-fishes 

 — Ostracion, the sun-fish — Orthagorisctis and others, are dis- 

 tinguished as Plectognathi. The body is globular or com- 

 pressed sideways ; the skin bears bony scutes or spines or is 

 naked ; the skeleton is incompletely ossified, and the verte- 

 brae are few ; the bones of the upper jaw are more or less 

 fused ; the pelvic fins are absent or reduced to spines ; the gills 

 are comb-like ; the swim-bladder has no duct. 

 It is likely that some of the loosely-built deep-sea fishes, such as the 

 pelican fish Eurypharynx are not referable to the orders usually recog- 

 nised. But the great majority of living fishes may be classified as 

 follows : — 



