58 FISHES OF ILLINOIS 



ORDER APODES 



THE EELS 



Body eel-shaped; skeleton bony; vertebrae numerous, the anterior ones 

 distinct, without Weberian ossicles: ventral fins absent; all fins without spines; 

 pectoral arch, if present, not connected with, and remote from, the skull; 

 mesocoracoid absent; opercular bones small and concealed; premaxillaries 

 absent maxillaries persistent in some forms (Anguillidce) ; air-bladder, if 

 present, communicating with oesophagus by an open duct. 



The eels are elongate serpentine fishes, mostly with naked 

 skin, or with extremely small imbedded scales. Their origin is 

 unknown. They show some kinship with the Isospondyli (shad- 

 and herring-like forms), from which they may have sprung by 

 degradation, though this is by no means certain. The forms 

 without paired fins are mostly marine. There are several 

 families, of which one is represented in American fresh waters. 



FAMILY ANGUILLIDCE 



THE TRUE EELS 



Body serpentine, or eel-shaped, covered with very fine scales which are 

 deeply imbedded in the skin; head naked; lateral line present; skeleton 

 osseous; vertebrae numerous, the anterior ones not modified; ventral fins 

 absent; no spines in fins; dorsal and anal continuous with caudal around tail, 

 which is isocercal (i. e., with the caudal vertebrae decreasing in size in a 

 straight line backwards, as in the Anacanthini) ; mesocoracoid absent; gill- 

 openings much restricted, about as wide as the base of the pectorals; oper- 

 culum small, concealed beneath skin; mouth terminal; jaws about equal; 

 premaxillaries absent; maxillaries .ateral, separated on median line by the 

 coalesced ethmoid and vomer; maxillary, mandible, and vomer with cardi- 

 form teeth; air-bladder with open duct; young passing through a larval 

 stage, the ribbon-shaped larva being known as Leptocephalus (a name first 

 used to designate these forms as a distinct genus of fishes). 



Fresh and brackish waters of most parts of the world, but 

 not found on Pacific coast of North America or in islands of 

 the Pacific. A single genus known. 



