FISHES. gg 



that, on the one hand, the Amiids are the most teleostean (or like the ordinary fishes) 

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

 ganoid-looking of the teleosts. The succession from those forms is tolerably straight in 

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

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

 plexity to the systematist. These are the N"ematognaths, the several types of apodal 

 fishes, and the small group of Opisthomes, including the two families, NotacanthidsB 

 and MastacembelidsB. The genetic relations of none of these are fully understood. 

 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 primitive 

 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 generalized type, 

 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 typical teleosts. 



Teleosts are those fishes which are predominant in the present epoch 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 ojDtic nerves, as well 

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

 may state that the optic 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 ojajDOsite valves. With these characters ag 

 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 by Johannes 

 Milller 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 Anaoan- 

 thini, while others groxiped them together. This arrangement, howevei-, was a very 

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

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

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

 cates with the (Esophagus by a tube or mouth (j>husa, bladder, stoma, mouth), while in 

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

 {phusa, bladder, and kleistos, closed). But even the availability of these characters 



