The crayfish is a freshwater crustacean that is well studied because of its size and abundance. Crayfish are structurally similar to lobsters, which are marine crustaceans. Crayfish, lobsters, crabs, and shrimp are decapods (DEK-uh-PAHDZ), or members of the order Decapoda. Decapoda means "10 feet," a name used because these crustaceans have five pairs of legs that are used for locomotion.
The crayfish's external structure is shown in Figure 36-7. The body is divided into two major sections: the abdomen and the cephalothorax (SEF-uh-loh-THAWR-AKS). The cephalothorax consists of two tagmata: the head, which has five segments, and the thorax, which has eight segments and lies behind the head. The dorsal exoskeleton over the cephalothorax is a single, tough covering known as the carapace (KAR-uh-PAYS). The abdomen, the tagma behind the cephalothorax, is divided into six segments.
A pair of appendages is attached to each segment of the crayfish. Several pairs have specialized functions, as summarized in Table 36-1. The two pairs of antennae include the branched antennules, which serve as feelers sensitive to touch, taste, and balance. The long antennae are also feelers that respond to touch and taste. Crayfish use a pair of mandibles to chew food and use two pairs of maxillae (maks-IL-ee) and three pairs of maxillipeds (maks-IL-i-PEDS) to manipulate food. The posterior pair of maxillae also function in respiration, and the maxillipeds are sensitive to touch and taste. The most anterior pair of walking legs, the chelipeds (KEE-luh-PEDS), end in large pincers used for capturing food and for defense. The other four pairs of walking legs carry the crayfish over solid surfaces; two of these pairs end in small pincers that can grasp small objects. The swimmerets, which are attached to the five anterior abdominal segments, create water currents and function in reproduction.
Ventral nerve cord
Walking leg figure 36-8
At the posterior end of the crayfish is a paddle-like tail made up of the telson and the uropods (YOOR-oh-PAHDZ), which are attached to the sixth abdominal segment. Powerful abdominal muscles can propel the animal rapidly backward in a movement referred to as a tail flip.
The major parts of the digestive tract of a crayfish are shown in Figure 36-8a. Food passes through the esophagus to the stomach, where teeth made of chitin and calcium carbonate grind the food into a fine paste. After the paste is mixed with enzymes secreted by a digestive gland near the stomach, it enters the intestine for further digestion and absorption. Waste leaves through the anus.
Like most crustaceans, crayfish have featherlike gills for respiration. Figure 36-8b shows that the gills extend from the base of each walking leg into a chamber under the carapace. As a crayfish walks, its legs circulate water across its gills. Feathery branches on the posterior pair of maxillae also help direct water over the gills. Each gill is covered by an extension of the exoskeleton that is thin enough to permit gases to diffuse across the gill surface.
The main components of the crayfish's open circulatory system are shown in Figures 36-8a and 36-8b. In this system, the dorsal heart pumps a circulatory fluid called hemolymph into several large vessels that carry the fluid to different regions of the body. The fluid leaves the vessels and enters spaces within the body, where it bathes the various tissues. It then passes through the gills, where it exchanges carbon dioxide with oxygen in the water. From the gills, the fluid returns to the dorsal part of the crayfish and enters the heart.
Ventral nerve cord
Walking leg figure 36-8
The major internal organs of a crayfish are seen in this cutaway side view (a) and cross section through the heart region (b).
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As freshwater organisms, crayfish live in a hypotonic environment. Remember that a hypotonic environment is one in which the concentration of solute molecules is lower than that in the organism's cells. Therefore, water constantly enters the tissues of a crayfish by osmosis. This excess water, along with wastes, is eliminated by excretory organs called green glands, which are visible in Figure 36-8a. The dilute fluid collected by the green glands leaves the body through a pore at the base of the antennae.
The nervous system of the crayfish is illustrated in Figure 36-8a. It is typical of arthropods and is similar to the nervous system of annelids. The crayfish brain consists of a pair of ganglia above the esophagus that receive nerve impulses from the eyes, antennules, and antennae.
Two bundles of nerve fibers extend from the brain and pass around either side of the esophagus to a ganglion that controls the mandibles, maxillae, and maxillipeds. The ventral nerve cord runs posteriorly from this ganglion, connecting a series of ganglia that control the appendages and muscles in the segments of the thorax and abdomen.
Crayfish sense vibrations and chemicals in the water with thousands of small sensory hairs that project from the exoskele-ton. Sensory hairs are visible in Figure 36-1. These sensory hairs are distributed over the entire body, but they are especially concentrated on the antennules, antennae, mouthparts, chelipeds, and telson.
The compound eyes of a crayfish are set on two short, movable stalks. Each eye has thousands of light-sensitive units, each with its own lens. At the base of the antennules are organs that can detect the animal's orientation with respect to gravity.
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