58 FISHES OF ILLINOIS 



Order APODES 



(the eels) 



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

 ones distinct, without Weberian ossicles r 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 con- 

 cealed; premaxillaries absent; maxillaries persistent in some forms 

 {Anguillidm); 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 Isuspondyli (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 ANGUILLIDjE 



(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; skele- 

 ton 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 Anacanlhini) ; mesocoracoid 

 absent; gill-openings much restricted, about as wide as the base of the 

 pectorals; operculum small, concealed beneath skin; mouth terminal; 

 jaws about equal; premaxillaries absent; maxillaries lateral, separated on 

 median line by the coalesced ethmoid and vomer; maxillary, mandible, 

 and vomer with cardiform 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). 



Preshand 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. 



