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Horseshoe crabs are represented in the World's Oceans today by three genera (main types): Limulus polyphemus, Carcinoscorpius
rotunda and Tachypleus gigas.
The photograph on the left shows the main characteristics of the dorsal (or top)
surface of the horseshoe crab Limulus polyphemus, which occurs on the North-east seaboard of the United States of America.
Towards the front end (anterior) of the animal is the domed carapace. Limulus has eyes positioned either side of
the midline of the carapace as well as light sensitive areas in the centre. The shovel-shaped carapace reflects its main use
as a digging tool, allowing Limulus to dig shallow pits both in search of food items such as clams, excavate nests for eggs
and hide itself below the sand.
The main body portion (opisthosoma) acts as a protective cover for the gills or
respiratory system. At the end of the body is a strong, articulated tail spine. The tail spine is used by Limulus as a self-righting
mechanism if the animal is overturned accidently. The movable spines which stick out from the sides of the main body are used
as sensory structures to let the animal know water current strength and direction.
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This picture illustrates the ventral or underside view of the same individual. The first thing to notice is how different
the appendages are on the head region and the body region. The head region houses walking leg appendages just as in all other
chelicerates such as spiders and scorpions. The first pair, which are very small and elongate are termed the chelicerae and
are used to manipulate food particles towards the mouth. The remaining pairs are used in locomotion, to move the animal across
the sea floor. The last pair of legs are well adapted to this and are often termed "pushers". These dig below the
sediment surface to gain purchase for movement.
The main body region houses a series of gills which are arranged
like the consecutive pages of a book, hence their informal name "book-gills".
Lastly, the strong articulation
point of the tail spine can be seen in this view. The tail spine can swivel around on this articulation when the animal is
righting itself.
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