Basic Anatomy
There are three divisions to the body of the horseshoe crab: the prosoma , the opisthosoma, and the telson. These are sometimes referred to as the cephalothorax, the abdomen, and the tail.
The prosoma contains a sizeable intestinal tract with an esophagus and proventriculus (used to grind food), a nervous system concentrated into a bulbous brain, a tubular heart, excretory glands at the bases of the walking legs and connective tissue and cartilagenous plates.
The opithosoma contains chiefly the musculature for the operation of the book gills and the telson, though the horseshoe's 113 distinct muscle groups (comprising over 750 individual muscles) are not limited to this section of the body.
Gauvry, Glenn. "Topographical Characteristics."
Horseshoecrab.org, 2003,
www.horseshoecrab.org/anat/anat.html
Gender
To join the survey, it is necessary to correctly identify female and male horseshoe crabs.
The easiest way to identify a female or male horseshoe crab is by the second pair of legs. A look at the underside of the horseshoe reveals six paired appendages. The horseshoe uses the first pair (the chelicera) for placing food in its mouth. The next pair of appendages are the pedipalps; these are the first ambulatory legs. In the adult male, the tarsus of these legs are modified as a grasping appendage, allowing males to clasp the female during spawning. This set of legs on the male horseshoe crab are said to resemble boxing gloves. While the female horseshoe crab has all similar legs expect for the rear pair that are known as "pusher" legs.
Without interrupting spawning, it is possible to determine the gender of the horseshoe crab by the size and location. The females are larger in size than the males and are usually buried in the sand with one or more males surrounding her.
Male Horseshoe Crab.
Notice the clasper front appendages used to clamp onto the female during spawning.
Female Horseshoe Crab.
The female horseshoe crab does not have front claspers, instead the second pair of appendages are the pedipalps; these are the first ambulatory legs that look similar to the other ambulatory legs.