FISHES. 



99 



that, Dii the OIK' haiul, tlie Auiiids are the most teleostean (or like the ordinary fishes) 

 of the ganoids, and, on tlie other, such forms as tlie Albulids and Elopids are tlie most 

 ganoid-looking of the teleosts. The suecession from those forms is tolerably straiijht in 

 their varions ramifications to the ends of the different series. But there are several 

 types that break into the continuity of the series, and arc a source of trouble and per- 

 plexity to tiie systematise These are the Nematognaths, the several types of ajHHlal 

 fishes, and the small group of Opisthomes, including the two families, Notacantliida' 

 and Mastacembelida'. The genetic relations of none of these are fully undei'stftod. 

 By several authors — among them Agassiz and Cope especially — the group of Nema- 

 tognaths, or catfishes and their relations, has been regarded as most nearly related to 

 the sturgeons, and by Cope it is imagined that "future discoveries will prove that it 

 has been derived from that division by descent." To others this relationship is not 

 perfectly clear. The Nematognaths are, however, doubtless modified from a ])riniitive 

 type not in line with the majority of existing fishes, and it will be convenient to con- 

 sider them among the first, although not the very first. There are the various eel-like 

 forms, whose relations are as little, if not still less, understood than those of the Nema- 

 tognaths. It is evident, however, that the eels have sprung from a gcnei-alized tyjie, 

 and apparently also from a stock out of the line of the typical fishes, although, perhaps, 

 from a more recent type than the ancestors of the catfishes. Nevertheless, it will in- 

 terfere less with a serial arrangement if we commence with the eel-like forms and 

 consider their ramifications, after which we may take up the Nematognaths and their 

 kindred, and finally the more ty])ical teleosts. 



Teleosts are those fishes which are predominant in the present e])o(-h of our globe, 

 and which are mostly characterized by a well-ossified skeleton ; and in allusion to this 

 feature the sub-class name has been given (teleios, perfect, osteon, bone). The skeleton, 

 however, is as much ossified in certain Ganoids {Lepidosteus and Amia) as in some 

 Teleosts, and consequently we have to study other parts of the organization to learn 

 whether the group is a natural one. As a result of such study, it has been found that 

 there is a considerable departure in the brain, and especially the o[)tic nerves, as well 

 as in the heart, from the types heretofore considered. Without going into details, we 

 may state that the oj)tic nerves simply decussate or cross each other without any 

 blending of fibres or chiasma, as in the Ganoids, and the heart has in front an arterial 

 bulb, which has generally only a pair of opposite valves. With these characters as 

 key-notes, we can co-ordinate others, and convince ourselves that the Teleosts have 

 developed through an essentially continuous though perhaps composite line from a 

 Ganoid stock; for, diversified as they are, the capital characters differentiating sub- 

 divisions are few, and the intergradation manifest. It is indeed a difficult matter to 

 find gaps of demarcation. For many years the grouping sanctioned liy .Johannes 

 Miiller has been a popular one. In this the Lophobranchiates and Plectognaths were 

 set apart, and all the rest were classified as Physostomes and Physoclists, some divid- 

 ing the latter into ' orders,' designated as Acanthopteri, Pharyngognathi, and Anacan- 

 thini, while others grouped them togethei-. This arrangement, however, was a very 

 artificial as well as a very superficial one, for unlike forms were united while like 

 ones were with equal violence divorceil. There was, nevertheless, a shade of reason 

 in the grouping. The Physostomes were those Teleosts whose air-bladder communi- 

 cates with the oesophagus by a tube or mouth (phusa, bladder, stoma, mouth), while in 

 the Physoclists the air-bladder is shut off from all communication with the outer world 

 (2)husa, bladder, and kldstos, closed). But even the availability of these characters 



