What does a horseshoe crab look like?

Horseshoe crabs are large animals as invertebrates go. Females can be up to 60 cm in length and can weigh up to 5 kg. They resemble large horseshoe shaped helmets with a long spike-like tail or telson. The carapace is smooth and of a brown or tan color. In some environments the crab can be covered with so much epiphytic and epizooic growth that the carapace itself is not visible. The shape of the carapace facilitates pushing through sand and mud and also protects the more vulnerable underside of the animal.

Two large compound eyes are visible on the top of the animal and resemble a large version of the compound eyes of flies and butterflies. These compound eyes are made up of units of 8 to 14 retinular cells grouped around a rhabdome. Each of these units has a lens and cornea and is known as an ommatidium. The compound eye is therefore a group of ommatidia. The relatively simple arrangement of these ommatidia means that the crab is not able to form very distinct images. It also means that the relationship between light stimulus and what happens in the nervous system can be studied very effectively. That, coupled with the large size and relative abundance of this animal made it an ideal model for studying the basis of how eyes work.

The carapace forms the bulk of the animals size. It forms a crescent around the next section, the abdomen. The margin between the two forms a hinge that closes inward to protect the underside of the body when the crab is alarmed. The abdomen possesses 6 pair of spines along the margin which further protect the underside. A line of small indentations run along the median of the abdomen in pairs and represent attachment points for muscles which control the movement of the gills.

The body ends in a long spine called a "telson" which can be rotated by the crab and is used to right itself when it is accidently (or intentionally) flipped over. The telson is not used for defense and, aside from being hard and pointed, is not dangerous. Many people pick individuals up by their telson.

The underside of the crab carapace is taken up by five pairs of walking legs and the chelicera. (see how horseshoe crabs eat for more) The walking legs end in chelae ("key-lay"- meaning claws) which can grip but are not at all capable of breaking the skin. The first segment of the walkings legs is called the coxae and in the first four sets of legs these are modified with large bristly spines used for feeding. The coxa in the fifth pair or legs bear a short spatula-shaped paddle which is used for cleaning the gills. The fifth pair of legs also contains several leaflike entensions which the crab uses for pushing and clearing mud and silt when it burrows.

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