Biology-Mr.Willets
Lab-Crayfish
The crayfish (Cambarus), is a member of Phylum Arthropoda, Class Crustacea. This phylum is the most successful of all animal groups having more than 800,000 species. The most noticeable characteristics of arthropods are jointed appendages and a hard exoskeleton. Both of these characteristics are obvious in the crayfish. The large chart and the books (pg 427) can help you identify the parts in boldface.
EXTERNAL ANATOMY
Examine your specimen, dorsal side up. You can easily see that the body has two main parts: the posterior abdomen (you might call this the tail) and the anterior cephalothorax. This front area is really two sections (head & thorax) that have been joined and therefore act as a single piece. The line where they join is obvious. Although the cephalothorax is made of two parts, they are connected so that they move as one piece.
You can see that the abdomen is divided into segments. This shows a relationship to what other phylum?
Locate the compound eyes. These are on stalks. Compound eyes have many lenses, each of which produces an image. Use the hand lens to examine the eyes. Look for the lenses. What shape is each lens?
There are two pair of sensory appendages located at the anterior end. The smaller pair are called antennules. The larger pair are the antennae.
Examine the ventral surface and find the mouth. It is surrounded by several pairs of appendages collectively known as the mouth parts. These include: 1) mandibles or true jaws- these are hard and tooth-like 2) maxillae- there are 2 pair of these to help the mandibles 3) maxillipeds- these "jaw-feet" hold the food while the crayfish is chewing. Crayfish are scavengers and eat whatever is laying around. You will have to poke around in the mouth area to find all these appendages.
The most obvious pair of appendages are the chelipeds with well developed pincers for getting food and for protection. Notice how each joint of this leg is a hinge joint and can only bend in one plane. However, the combination of all the joints in the leg operating in different planes gives the leg a wide range of motion. Play around with it to see for yourself.
There are 4 pair of walking legs posterior to the chelipeds. Attached to these legs are the gills. The gills are located under the exoskeleton. When eating a lobster, you might pull off one of these legs and see the feathery gills attached. Look under the exoskeleton to see the gills. This part of the skeleton can pump water over the gills. The water moves from posterior to anterior.
The appendages along the ventral surface of the abdomen are called swimmerets. In females, the first pair of these is soft and like the others. In males, the first pair are larger and rigid. These are used by the male to transfer sperm to the female. Make sure you see a crayfish of each sex. The swimmerets of the female are used to hold the fertilized eggs in place while they develop.
The last segment on the abdomen is a triangular piece called the telson. To each side of the telson are appendages called uropods. When the uropods spread out, they form a "fan" which the crayfish can use to propel itself backwards when in danger. Spread these apart to see this fan. The anus can be seen as a slit like opening on the ventral side of the telson.
You will notice sensory bristles all over the crayfish especially on its appendages. These react to touch and perhaps sound waves.