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The smallest species is the mangrove horseshoe crab (C. rotundicauda) and the largest is the tri-spine horseshoe crab (T. tridentatus). [ 44 ] On average, males of C. rotundicauda are about 30 centimeters (12 inches) long, including a telson that is about 15 cm (6 in), and a carapace about 15 cm (6 in) wide. [ 45 ]
The horseshoe crab has traditionally been used in investigations into the eye, because it has relatively large ommatidia with large nerve fibres (making them easy to experiment on). It also falls near the base of the chelicerates ; its eyes are believed to represent the ancestral condition because they have changed so little over evolutionary time.
A 2021 study found that the upper limb branch of trilobites is a "well-developed gill" that oxygenates the hemolymph, comparable to the book gill in modern horseshoe crab Limulus. In Olenoides , the partially articulated junction with the body is distinct from the exopods of Chelicerata or Crustacea .
Merostomata is a class of chelicerate arthropods that contains the extinct Eurypterida (sea scorpions) and the extant Xiphosura (horseshoe crabs). The term was originally used by James Dwight Dana to refer to Xiphosura only, but was emended by Henry Woodward to cover both groups.
The group has hardly changed in appearance in hundreds of millions of years; the modern horseshoe crabs look almost identical to prehistoric genera and are considered to be living fossils. The most notable difference between ancient and modern forms is that the abdominal segments in present species are fused into a single unit in adults.
The tri-spine horseshoe crab is the largest of the living horseshoe crab species. [8] Like the other species, females grow larger than males. The largest females of the tri-spine horseshoe crab can be as much as 79.5 cm (31.3 in) long, including their tail. [9]
Atlantic horseshoe crab on the shore at Brighton Beach, New York City. The Atlantic horseshoe crab (Limulus polyphemus), also known as the American horseshoe crab, is a species of horseshoe crab, a kind of marine and brackish chelicerate arthropod. [1] It is found in the Gulf of Mexico and along the Atlantic coast of North America. [1]
Horseshoe crab blood contains a clotting agent, limulus amebocyte lysate, which is used to test antibiotics and kidney machines to ensure that they are free of dangerous bacteria, and to detect spinal meningitis and some cancers. [94] Cooked tarantula spiders are considered a delicacy in Cambodia, [95] and by the Piaroa Indians of southern ...