TUBE-BLADDERED FISHES EELS AND ALLIES 2817 



THE TUBE-BLADDERED FISHES SUBORDER Physostomi 



It has been already stated on p. 2702 that the whole of the four preceding sub- 

 ordinal groups of the bony fishes are regarded by Professor Cope as constituting but 

 a single suborder (Physoclysti) characterized by the absence of a duct to the air 

 bladder, the separation of the parietal bones of the skull by the supraoccipital, and 

 by the pelvic fins being usually thoracic or jugular in position. The group to which 

 we now come, including the whole of the remaining representatives of the existing 

 bony fishes, differs from the above in that the air bladder, when present, has a duct 

 communicating with the stomach or oesophagus, while the pelvic fins are always 

 abdominal in position, and the parietal bones are usually in contact with each other. 

 With regard to the constancy and importance of these characteristics of the present 

 suborder, Professor Cope writes that the presence of the duct from the air bladder 

 which characterizes it, "is always associated with an abdominal position of the 

 pelvic fins and cycloid scales, and mostly with the presence of the precoracoid arch, 

 the entrance of the maxillary bone into the border of the mouth, and the non- 

 separation of the parietal bones by the supraoccipital. Yet none of these character- 

 istics are precisely associated at the point of change in each, for there are Physosto- 

 mous fishes with separated parietals and ctenoid scales (some Cyprinodontidce) , and 

 there are Physoclysti with abdominal pelvic fins." In the present suborder, with 

 the exception of the first in the dorsal and pectorals, which may be ossified into 

 spines, all the fin rays are soft and jointed. Very different views obtain as to the 

 best mode of arranging the families constituting the suborder, and a final classifica- 

 tion is still a desideratum. By Professor Cope the families have been arranged in a 

 number of sectional groups, mainly distinguished by the structure of the skeleton; 

 and a modification of this arrangement is adopted here, although fewer groups are 

 recognized. It is, however, impossible to enter here into the consideration of the 

 osteological features by which these sections are distinguished, and we are accord- 

 ingly compelled to rely mainly on external characteristics. 



THE EELS AND THEIR ALLIES Families 



STMBRANCHID^, and GTMNOTID^. 



The whole of the members of these three families are characterized by the 

 elongated, "eel-like" form of the body; but it is quite probable that this external 

 similarity is due to parallelism in development, and that the three families have 

 been independently derived from very different types of more normally formed 

 fishes. The first family, which includes the true eels, mursenas, and congers, is 

 characterized by the normal structure of the upper jaw, which is formed in front by 

 the premaxillse (more or less confluent with the vomer and ethmoid) and laterally 

 by the toothed maxillae. The median fins, when present, are either confluent or 

 separated by the projecting tail; the pectorals may or may not be developed; but 

 the pelvic pair is invariably wanting. There are no accessory breathing organs; the 

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