Circulatory system:
The horseshoe crab has a developed circulatory system.  A long heart runs down the middle of the prosuma ( and abdomen.  The rough outline of the heart is visible on the exoskeleton and at the hinge.  Blood flows into the book gills where it is oxygenated in the lamellea (a section of each gill).  The flapping movement of the gills circulates blood in and out of the lamellea. Oxygenated blood is returned to the heart for distribution through out the horseshoe crab.

Food:
The horseshoe crabs favorite foods are worms, mullusks, and dead fish.  When it sees a worm or clam the small claws pick it up and moves it to a bristly area near the back of the walking legs.  The horseshoe crab has no jaws an uses their bristles to crush the food as it moves its legs.  A horseshoe crab can only eat as it walks along the bottom.

Gills:
A horseshoe crab absorbs oxygen from the water using gills that are divided into 5 distinct pairs located under the abdomen.  Each pair of gills has a large flap-like structure covering lamellae (leaf-like membranes).  Each gill contains approximately 150 lamellae that appear as pages in a book.  The gills also function as paddles to propel juvenile horseshoe crabs through the water.

Legs and Appendages:
Beneath the shell are seven pairs of appendages (a part or organ). The first pair, called chelicerae, are small, and are used to push food into it's mouth. The mouth is located at the base of the legs.  Their are five pairs of long walking legs.  The last set of appendages, the chelaria, are also small, and are used to help the first pair move food towards it's mouth.  Each pair of legs has a small claw at the tip except the last pair.  The last pair of legs has a leaf like structure at the terminal end that is used for pushing and clearing away sediments as the crab burrows into marine bottom.  The base at each leg is covered with inward pointing spines called gnathobases that move food towards the mouth.  As the legs are moving, food is crushed and macerated( to soften).