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12 times, what's left passes into the midgut (whose function isn't really understood) then to the hindgut and eventually out the hind end. The circulatory system of the crayfish is an open system where the blood is contained in vessels for only part of the system. The heart is located in a pericardial sinus located in the upper part of the thorax (a sinus is a sac or cavity). The heart pumps the blood into the arteries. Anteriorly, one pair of arteries, the ophthalmic, carries blood forward to the eyes, the brain and the antennae. Another pair of arteries, the hepatic, carry blood to the hepatopancreas and the stomach. Posteriorly, the dorsal abdominal artery, feeds blood to the abdominal muscles and intestine. Just to the rear end of the heart, the sternal artery, drops down and supplies the ventral abdominal and ventral thoracic arteries which feed blood to the appendages and nerve cord. After leaving the arteries, the blood bathes the cells and organs, eventually collecting in a large sternal sinus in the bottom of the thorax. From here it passes through the gills and back to the pericardial sinus and then through three small valves back into the heart to be recycled. The nervous system mainly consists of a ventral nerve cord that has numerous swellings or ganglia. From the ganglia, nerves branch out laterally leading to the appendages and muscles. In the cephalothorax, the nerve cord leads to an enlarged ganglion that serves as a "brain". Nerves lead from the brain to the eyes, antennae and antennules. Their eyes are compound eyes on the ends of moveable stalks, each having thousands of facets. The reproductive system consists of pairs of testes or ovaries located in the upper rear part of the thorax between the hepatopancreas and the heart. In the female, the eggs pass down the oviduct to an opening at the base of the third walking legs. In the male, a pair of ducts (vas deferens) carry sperm to openings at the base of the rearmost walking legs. The vas deferens also packs the sperm into packets called spermatophores for later transfer to the female. The excretory system consists of the two green glands or antennary glands whose openings are just below the base of each antenna. The green gland filters waste out of the blood and feeds it into a bladder where it then exits through a pore at the base of the antenna. The urine is very dilute as these organs also function to get rid of the excess water that constantly floods the tissues of freshwater animals.