Fact of the Day : In which two areas of the world can horseshoe crabs be found? (from The Encyclopedia of Underwater Life )
Horseshoe Crabs
The subphylum chelicerata, of which horseshoe crabs form a small part, is more commonly represented by its terrestrial members – the arachnids (spiders, scorpions, and their relatives). The horseshoe crabs'primordial appearance hints at the fact that they form one of the most ancient surviving groups of organisms.
All chelicerates are typified by having a pair of pincerlike mouthparts (chelicerae), which are positioned in front of the mouth opening. They have no biting jaws, and two distinct parts of the body are recognizable, the prosoma (front part) and opisthosoma (rear part). Since horseshoe crabs fit this description and in spite of their common names and sea-dwelling habit, they are therefore more closely related to terrestrial arachnids than to crustaceans.
They have remained more or less unchanged for some 300 million years, since the Permian period. The fact that extant species occur in two widely separated areas of the world (the eastern seaboard of the USA, and South and Southeast Asia) suggests that horseshoe crabs are relicts of a far more extensive distribution.
More..http://www.oxfordreference.com/pub/views/fact-of-the-day.html?date=2011-09-03
