EEL-LIKE FORMS 



173 



ing organ, it is most highly specialized. Its group is an 

 interesting one, and has been but little studied. 



The Eel (Fig. 180) might well be taken as one of the 



Fig. 180. — The eel, Avguilla vulgaris, Turton. X J. (After GOODE in U. S. 

 F. C.) Europe, South Asia, North Africa, North America. 



fish forms evolved by special environment. Living in soft 

 river bottoms, a serpent-like movement in progression has 

 gradually been acquired ; its form has, therefore, become 

 elongated and rounded, and the internal structures corre- 

 spondingly modified. Fin structures have accordingly been 



~^. 



Fig. 181. — The perch, Perca americana { = JiuviaiiUs ?) , Schrank. X |. (After 

 GoODE in U. S. F. C.) 



metamorphosed, ventral fins lost, tail degenerated, and a 

 continuous dorsal and ventral secondarily evolved ; scales 

 have become reduced in size, supplanted by mucous layers. 



