Merostomata (classic) or Xiphosura (cladistic), or horseshoe crabs (common)

Body tagmata?

Unique Features--Distinguished by book gills and telson (class or group distinctions).

Aquatic (most marine, but a few fossil brackish and freshwater forms), but can come onto land for short periods of time.

Fossil record from Silurian (445 million years ago) onward
Early forms and larvae of modern forms are VERY trilobite-like.

 

Horseshoe crabs feed on the sea bottom, capturing small molluscs and worms with their pincers. They break up hard material, such as mollusc shells, by grinding it between the hard edges of their upper legs or the gnathobases. The chelicerae, or the chilaria, small degenerate legs located behind the pusher legs, can push any food into the crab's mouth. A horseshoe crab also has a gizzard that contains sand and small bits of gravel to help grind its food.

 

 

In horseshoe crabs, fertilization is external. During the spring breeding season, in the North American species, thousands of the animals come ashore to breed in shallow water. Each male attaches to a female and is dragged around until she is ready to lay eggs. Young horseshoe crabs return to the water as free-swimming larvae.

 

 

Everything you ever wanted to know about horseshoe crabs http://www.horseshoecrab.org/

 

Read the following webpage which describes the use of Horse shoe crabs in research

On web page reference is made to obtaining blood from the pericardial cavity.