Digestive systems mechanically and chemically degrade food into small molecules that can be absorbed, along with water, into the internal environment. These systems also expel the undigested residues from the body. Incomplete digestive systems are a saclike cavity with one opening. Complete digestive systems are a tube with two openings and regional specializations in between. Structural variations in bills, teeth, and regions of the gut are adaptations that allow an animal to exploit a particular type or types of foods.
Digestive system (Tract)
The digestive tract is the area where digestion takes place.
• The digestive tract varies widely in complexity from species to species.
• The most primitive form found in nematodes is simply a tubular gut with no specialized features.
• The slightly more advanced digestive tract found in earthworms includes specialized areas for ingestion, storage, fragmentation, digestion, and absorption.
• The most advanced digestive tracts, found among the vertebrates, exhibit specialization on a much greater scale.