Xiphosids

Xiphosids are a group of marine arthropods belonging to the order Xiphosura. They are commonly known as horseshoe crabs, though they are not true crabs. Despite their name, horseshoe crabs are more closely related to arachnids, such as spiders and scorpions, than to crustaceans. The order Xiphosura contains only four extant species, all of which are found in the shallow coastal waters of the Atlantic Ocean and the Indo-Pacific region.

The most distinctive feature of xiphosids is their hard, horseshoe-shaped carapace, which protects their body. They possess a long, spine-like tail called a telson, which is used for righting themselves if overturned but not for defense or stinging. Xiphosids have multiple eyes, including two large compound eyes on the top of their carapace and several smaller simple eyes. They breathe using book gills located on the underside of their abdomen.

These ancient marine creatures have a long fossil record, with their ancestors dating back over 450 million years. They are often referred to as "living fossils" due to their remarkable similarity to their ancient relatives. Xiphosids play an important role in their ecosystems, serving as a food source for various birds and fish. Their unique blue blood, containing hemocyanin, is also highly valued in the medical industry for its ability to detect bacterial endotoxins.