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Limulus: the living xiphosuran
Fossil and Living Horseshoe Crabs

Dorsal view of Limulus

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.

Ventral Limulus

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.

Limulus is well adapted to life on the sea floor as a benthic scavenger and predator. this explains its persistance through the fossil record and the many mass extinctions to the forms which live in the World's oceans today.