Pair of uropods
Crayfish, also known as crawfish, crawdads, freshwater lobsters, or mudbugs, are freshwater crustaceans resembling small lobsters, to which they are related; taxonomically, they are members of the superfamilies Astacoidea and Parastacoidea. They breathe through feather-like gills and are found in bodies of water.  Purpose: Background: Like all crustaceans, a crayfish has a fairly hard exoskeleton that covers its body. As shown in the diagram on the next page, its body is divided into two main parts, the cephalothorax and the abdomen. Now turn the crayfish ventral side up. Observe the location of each pair of appendages from the ventral side. 9. On the last abdominal segment, observe a pair of pointed appendages modified into a pair of uropods. In the middle of the uropods, locate the triangular-shaped telson. In the abdomen, the first five segments each have a pair of swimmerets, which create water currents and function in reproduction. The sixth segment contains a modified pair of uropods. In the middle of the uropods is a structure called the telson, which bears the anus. The uropod and telson together make up the tail fan. The crayfish moves backward by forcing water forward with its tail fan. Fun Fact: 90% of all of the crayfish eaten in the world come from Louisiana. Crayfish are commonly used as food, bait, and pets. Like all crustaceans, a crayfish has a fairly hard exoskeleton that covers its body. As shown in the diagram, its body is divided into two main parts, the cephalothorax and the abdomen. The cephalothorax consists of the cephalic (or head) region and the thoracic region. In the middle of the uropods is a structure called the telson, which bears the anus. The uropod and telson together make up the tail fan. The crayfish moves backward by forcing water forward with its tail fan. Fun Fact: 90% of all of the crayfish eaten in the world come from Louisiana. Purpose: Background: Like all crustaceans, a crayfish has a fairly hard exoskeleton that covers its body. As shown in the diagram on the next page, its body is divided into two main parts, the cephalothorax and the abdomen. The cephalothorax consists of the cephalic (or head) region and the thoracic region. 3. Turn the crayfish with its DORSAL side upward, and locate the rostrum, which is the pointed extension of the carapace at the head of the animal shown in the diagram above. Beneath the rostrum locate the two eyes. Notice that each eye is at the end of a stalk. 4. Locate the five pairs of appendages on the head region. Crayfish. Bethany Grant. Crayfish also called Crawfish or Crawdad, are crustaceans in which most of them live in freshwater and a few live in brackish or salt water. Some live in burrows on land close to a source of water, all crayfish have the ability to borrow (Hyman 1996). The freshwater crayfish live in lakes and rivers In the abdomen, the first five segments each have a pair of swimmerets, which create water currents and function in reproduction. The sixth segment contains a modified pair of uropods. In the middle of the uropods is a structure called the telson, which bears the anus. 3. Turn the crayfish with its DORSAL side upward, and locate the rostrum, which is the pointed extension of the carapace at the head of the animal shown in the diagram above. Beneath the rostrum locate the two eyes. Notice that each eye is at the end of a stalk. 4. Locate the five pairs of appendages on the head region. The crayfish is an invertebrate which means it has no backbone. It is classified in a group called crustaceans. These kinds of animals usually have two sets of antenna and have many legs. They have an exoskeleton and molt. They are different than insects because of the have limbs and double sets of antenna. 
https://en.wikipedia.org/wiki/Crayfish http://www.smithlifescience.com/crayfishdissection.htm http://jrsowash.wikispaces.com/file/view/crayfish.instructor.pdf http://jrsowash.wikispaces.com/file/view/crayfish.instructor.pdf http://biologyjunction.com/crayfish_dissection.htm http://academics.smcvt.edu/dfacey/AquaticBiology/Freshwater%20Pages/CRAYFISH.html http://biologyjunction.com/crayfish_dissection.htm http://crayfishblue.weebly.com/index.html
what structure is modified in crayfish