80 CLASS IVe ORDER TI. 
creased probably from five to ten fold within the 
last century. Probably the encrease of animal 
substances, along the river and its tributary 
branches, which are perpetually washed into it, 
furnishes an immense addition to the stock of 
food suited to their natures. 
Most fish feed on both animal and vegetable 
substances. Some species will live many years 
in a vessel of mere water. Whether they possess 
the power of decomposing water and forming new 
compounds fit for food, or whether the water ab- 
sorbs organized matter from the atmosphere suf- 
ficient to support them, or on what other principle 
their lives are prolonged in this situation, is not 
determined. But the same difficulties present 
themselves in regard to the leach and numerous 
other avertebral animals ; excepting that the lat- 
ter have a less complicated organization. 

tral fins none. 
AwneuILua, (eel,) head smooth ; nestrils tubu- 
lar ; eyes covered by the common skin; gill-mem- 
branes 10-rayed; body roundish, smooth, mu- 
cous; dorsal, caudal, and anal fins united 5 spi- 
_racles (breathing holes) behind the head or pecto- 
Fal fins. °°" 
Muraena, (eel, serpent-eel,) body eel-form ; 
pectoral fins none ; spiracle on each side of the 
neck. 
* I prefer the orders of Linneus in this class, for reasons given in the 
preface. Cuvier says the orders of fish are more difficult to define than 
the orders of any other class of animals. 
