9) Refer to Figure 4 and identify the organs of the circulatory system. Locate the dorsal tubular heart and several arteries. The crayfish has an open circulatory system in which the blood flows from the arteries into sinuses, or spaces, in tissues. The blood flows over the gills before returning to the heart. The heart is within the pericardial sinus, or cavity, at the posterior edge of he cephalothorax. With the hand lens, observe the three pairs of ostia, or small openings, through which blood enters the heart. Observe as many arteries as you can without damaging the internal organs.
10) Refer to Figure 4 to locate and identify the organs of the nervous system. Find the ventral nerve cord that lies along the central midline of the crayfish. The thin plates of shell that partly cover the nerve cord in the cephalothorax make the cord diffficult to find in this area. Locate a ganglion, one of the enlargements, located on the ventral nerve cord.
>> Find the brain just above the esophagus. Carefully cut away the rostrum and any remaining carapace. Between the eyestalks is the brain, a small white mass. Note the nerves traveling from the brain to the eyes and antennae.
11) Locate the dorsal brain, which is located just behind the compound eyes. Note the two large nerves that lead from the brain, go around the esophagus, and join the ventral nerve cord. The compound eyes are made up of long visual rods. The outer surface of each rod is called a facet. Light is focused through each facet onto the retina, producing a fuzzy but wide-ranging image.
>> Remove an eye by clipping it at its base and examine it with the dissecting microscope. Note the numerous facets in the eye. Because the eyes are on movable stalks, the crayfish has a very wide field of view.
12) Refer to Figure 4 to locate and identify the organs of the digestive and excretory system. The blood carries cellular wastes to the disklike green glands. Locate these organs just in front of the stomach by carefully removing some of the muscle tissue in the anterior part of the cephalothorax. Locate the stomach, a large, thin-walled, two-chambered sac that is just above the mouth and esophagus. The large, yellowish-green digestive glands on either side of the stomach and part of the intestine excretes waste through pores at the base of each antenna.
>> Cut through the stomach, and notice in the anterior region of the stomach wall the heavy, toothlike projections, called the gastric mill, that grins up food. You should be able to see some grinding stones injested by the crayfish.
>> Using a probe, separate the digestive glands from the stomach to observe where the stomach joins the intestine. Follow the intestine along the length of the abdomen to the anus. The anus is located on the ventral surface of the telson.
13) Refer to Figure 4 to locate and identify the organs of the reproductive system. The gonad, either testis or ovary, is beneath and slightly anterior to the heart. The animal shown in the diagram is a male crayfish. If your specimen is a male, locate the testis. The testis is the long, white organ under the heart and a bit forward. The sperm ducts that carry sperm from the testis open at the fifth walking leg. If your specimen is a female, locate the bi-lobed ovary. It is in the same relative position as the testis, but the ovary appears as a large, reddish/pinkish/orange mass under the heart. Then locate the short oviducts that extend from near the center of each side of the ovary and open at the third walking leg.
>> Mating takes place in autumn. Sperm pass from the a male's testes through the ducts to the outside. Using the modified swimmerets, the male transfer his sperm to the female's seminal receptacle, where the sperm are stored over the winter. The eggs are not fertilized until the female lays them in April.
14) Exchange your specimen with a nearby classmate who has a crayfish of the opposite sex, if available. Then study its reproductive system.
