At the anterior end of the cephalothorax note the large membranous sac, the _stomach_. Attached to each end of this are sets of muscles which control its movements. To the right and left of the stomach notice attached to the shell large muscles which connect by stout ligaments at their lower ends with the mandibles. Note a yellow fringe-like structure, the _digestive gland_, which fills most of the region about the stomach. It connects by a pair of small tubes, the _bile-ducts_, with the alimentary canal. Within the posterior portion of the cephalothorax note a pentagonal sac, the _heart_, contained within a delicate membrane, the _pericardium_. Remove the pericardium and note a pair of dorsal openings into the heart, called _ostia_. (There are also two lateral pairs and a ventral pair of ostia.) Note passing anteriorly from the heart along the median line to the eyes a blood-vessel, the _ophthalmic artery_. Arising from the anterior portion of the heart are the _antennary arteries_, running to the antennæ. Yet another pair running anteriorly from the heart to the stomach and digestive glands are called the _hepatic arteries_. From the posterior end of the heart arises the _dorsal abdominal artery_, running back to the telson. Below this arises the _sternal_ artery, which will be seen later. In the region below the heart are located the reproductive organs. They are whitish glandular masses from each of which runs a tube which opens at the base of the last pair of walking-legs in the male, and at the base of the third pair of walking-legs in the female. TECHNICAL NOTE.--Cut longitudinally through the dorsal wall of the abdomen on either side of the median line and remove the piece of shell. Note the powerful _muscles_ within which flex and extend the abdomen. By a rapid contraction of these muscles the tail is brought beneath the body, propelling the animal strongly backwards. When the crayfish crawls it generally goes forward, but in swimming it reverses this direction. Make a drawing showing, in their natural position, the internal organs which have been studied. Examine the alimentary canal for its whole length. Note that the large bladder-shaped stomach is attached to the mouth-opening by a short tube. What part of the canal is this? From the posterior end of the stomach is a short thick-walled part, the _small intestine_, followed by a long straight tube, the _large intestine_, which opens to the exterior through the _anus_. TECHNICAL NOTE.--Remove the alimentary canal, detaching it from the anal end first, and working forward.