EEL-LIKE FORMS 



173 



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

 interesting one, and has been but little studied. 



The ^r/ (Fig. 180) might well be taken as one of the 



Fig. 180. — The eel, Anguilla vulgaris, Turton. X \. (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 aiiiericana {^ JLuviatUls?), 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. 



