Also known as Horseshoe Crabs, aka, those guys who decided about 450 million years ago...

And so they didn't.

The entire body of the horseshoe crab is protected by a hard carapace. It has two compound lateral eyes, each composed of about 1000 ommatidia, plus a pair of median eyes that are able to detect both visible light and ultraviolet light, a single endoparietal eye, and a pair of rudimentary lateral eyes on the top. The latter become functional just before the embryo hatches. Also, a pair of ventral eyes is located near the mouth, as well as a cluster of photoreceptors on the telson. Despite having a relatively poor eyesight, the animals have the largest rods and cones of any known animal, and their eyes are a million times more sensitive to light at night than during the day. The mouth is located in the center of the legs, whose bases are referred to as gnathobases and have the same function as jaws and help grinding up food. The horseshoe crab has five pairs of legs for walking, swimming, and moving food into the mouth, each with a claw at the tip, except for the last pair. The long, straight, rigid tail can be used to flip the animal over if turned upside down, so a horseshoe crab with a broken tail is susceptible to desiccation or predation. Behind its legs, the horseshoe crab has book gills, which exchange respiratory gases, and are also occasionally used for swimming. As in other arthropods, a true endoskeleton is absent, but the body does have an endoskeletal structure made up of cartilaginous plates that support the book gills.

Horseshoe crabs resemble crustaceans, but belong to a separate subphylum, Chelicerata, and are closely related to arachnids. Unlike vertebrates, horseshoe crabs do not have hemoglobin in their blood, but instead use hemocyanin to carry oxygen. Because of the copper present in hemocyanin, their blood is blue. During the breeding season, horseshoe crabs migrate to shallow coastal waters. A male selects a female and clings to her back. The female digs a hole in the sand and lays her eggs while the male fertilizes them. The female can lay between 60,000 and 120,000 eggs in batches of a few thousand at a time. The eggs take about two weeks to hatch; shore birds eat many of them before they hatch. The larvae molt six times during the first year.

However, while the body plan of true horseshoe crabs has served them well, they do have several other aberrant cousins.

BUMBLELAND STABBERS

The Stabbers are a group of Xiphosurids native to Bumbleland, though one species has extended its range across the northern coasts of the Grey Ocean, from the eastern coast of the Crystal Empire to the coast of Sweden.

Oceanic Stabber

Found across the northern coasts of the Grey Ocean, the Oceanic Stabber is a solitary species that often becomes hostile when provoked.

It is a fairly fast animal, able to quickly run down prey on land. In the water it tucks its limbs in to become more compact, allowing it to move at a fair speed while swimming.

Unfortunately, Oceanic Stabbers within Bumbleland are confined to the beaches, rivers, and lakes due to the presence of the native Dromaeosaurine Mantids who occupy a similar niche further inland AND have the advantage of both flight and not having to keep a set of gills moist in order to breathe.

Jungle Stabber

Within the Misty Forest of Bumbleland, the near-constant fog provides enough moisture to allow this smaller relative of the Oceanic Stabber to maintain its life away from water, though it still needs to return to the rivers to lay its eggs.

The Jungle Stabber's larger forelimbs and shorter claws are adapted to punch through the chitin of other arthropods found within the Misty Forest, but it faces the same problem as its coastal relative in the form of the Dromaeosaurine Mantids who live outside its jungle home.

BADLAND LURKERS

these Xiphosurids are native to Equus' Badlands, inhabiting the rivers and single marsh of the region

Mirelurk

Found around the few rivers in the Badlands, Mirelurks are semi-eusocial arthropods divided into four castes. However, this is not a true eusocial society like that found in ants, as all the females are fertile, as are the males.

The males are divided into three types.

The Mirelurks (center) are normally peaceful unless provoked. The Hunters (right) are far more aggressive, and are responsible for gathering food for the others. Finally, the Glowing Hunters (left) are more nocturnal.

Marshlurk

Found in the Badland's single swamp, the Marshlurks are a true Eusocial species, centered around a massive queen who lays all the eggs.