ZEL-LIKE FORMS 173 
ing organ, it is most highly specialized. Its group is an 
interesting one, and has been but little studied. 
The £e/ (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 
Pig. 181.— The perch, Perca americana (= fluviatilis ?), Schrank. x 4. (After 
GoopE 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. 
