River crayfish: internal structure
The crayfish’s muscles consist of separate bundles. Digestive system: mouth, pharynx, gullet, a two-chambered stomach (food is ground in the large one and strained in the small one), intestine and liver tubes; waste leaves through the anus in the middle of the tail fan. The heart lies at the back of the cephalothorax; circulation is open – blood pours into the body cavity and is then enriched with oxygen in the gills. The excretory organ is a pair of green glands in the head, their tubes opening at the base of the short antennae. The nervous system resembles the earthworm’s: supra- and sub-pharyngeal ganglia, a ring and a ventral nerve cord. The eyes are compound, made of very many tiny units – mosaic vision; hairs on the antennae sense smell and touch, and an organ at the base of the short antennae helps with balance. There are over 60,000 species of crustaceans: most live in the sea, and plankton such as Daphnia and Cyclops feed fish. Crabs, lobsters and shrimps are caught for food. The woodlouse has adapted to land, loosens soil and enriches it with humus.
Diagram: “Crayfish organs” – draw the digestive, circulatory, excretory and nervous systems in different colours. Safety: do not eat undercooked seafood; do not go to riverbanks without adults; you may pick up a woodlouse but wash your hands after.