Blue crab eyes allow seeing 360 degrees giving them a good view of their entire surroundings. Crabs can see polarized light. Their eyes are on stalks which can be pulled into protective grooves when the crab burrows into the muddy bottom. Additionally crabs have two sets of antennae. Both pairs of antennae are small. The lateral pair is the second antennae. Each consists of a basal peduncle of three articles whose proximal article is fused with the carapace and bears a small, heavy, transverse ridge.
Anterior to the first maxillae are the large, hard mandibles. Each one consists of a heavily calcified protopod from which arises a small palp. The protopod is divided into a medial cutting service and a large basal region for the insertion of the muscles that operate the mandible.
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The posteriormost head appendage is the SECOND MAXILLA(below) lying immediately anterior to the first maxilliped. One of the several parts is a rectangular gill bailer which generates the respiratory current through the branchial chamber. The first maxilla is even smaller and has two endites and an endopod but no exopod.
The 3 pairs of the maxillipeds are the appendages of the first 3 thoracomeres and are biamous.
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The maxillipeds and other mouth parts overlie each other so only MAXILLIPED 3(below) can be seen. The two exhalant apertures through which water exits the branchial chambers are not as obvious as the inhalant aperture but is located lateral to the first and second maxillipeds. Water can exit these openings even when the third maxillipeds are closed over the mouth.
The nervous system is transparent and very hard to see. The nervous system of brachyuran crabs is highly cephalized into a brain and a large thoracic ganglion.
Females have paired biramous pleopods on abdominal segents 2-5 but they are hidden under the flexed abdomen which must be extended to reveal them. The exopod and endopod arise from the basis. After release from the gonopores, the eggs attach to the long setae of the pleopods where they are ventilated by movements of the abdomen and the pleopods. The 5 pairs of pereopods, or walking legs, of the posterior thorax do not have exopods. They are slender stenopods consisting of the endopod and protopod.
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Thanks to Richard Fox, Professor of Biology, Lander University, Greenwood, SC., for allowing use of the drawings and most descriptive materials on this page. You are encouraged to visit his
Website at: http://webs.lander.edu/rsfox .
DEFINITIONS:
Peduncle -In anatomy, a cerebral peduncle is a band of neurons, resembling a stalk
Transverse-Extending or lying across; in a crosswise direction; at right angles to the long axis
Mandibles-The mandible (from Latin mandibūla, "jawbone") or inferior maxillary bone is, together with the maxilla, the largest and strongest bone of the "face"
Protopod-A part of a crustacean appendage
Palp-To have a distinct touch or feeling of; to feel
Exopod-The outer branch of a crustacean limb
Thoracomer-Part of crustacean morphology
Biamous-An exopod, or typical crustacean appendage
Ganglion-Cluster of nerve cells: a structure that contains a dense cluster of nerve cells
Stenopod-Stenopods, meaning "narrow foot," are the typical appendages of most of the larger crustaceans such as crabs, shrimps
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