Internal anatomy of a crayfish: edible freshwater crustacean, with pincers on the two forelegs. Encephalon: site of the mental functions of a crayfish. Stomach: part of the digestive tract between the esophagus and the intestine. Heart: blood-pumping organ of the crayfish. Gonad: sex gland of a crayfish. Extensor muscles: muscle that extends the tail of the crayfish. Anus: outlet of the digestive tract.