Typically better developed in more carnivorous species. May serve for secretion, absorption or both. Produces various enzymes. Coiled many times, longer than body in herbivores 2 to 15 X body length. Functions in digestion, absorption and water resorption.
Fish have enzymes suitable for breaking down their particular foods.
Other foods, even though ingested, may not be digested. No fish is known to produce cellulase although some show bacterial cellulase activity in intestine. Rainbow trout sometimes eat large quantities of the filamentous alga, Cladophora, but lack either grinders or cellulase. The algae comes out about the same as it went in. Some planktivores pass ingested algae right through, sometimes still alive.