The Living Fossils.

These are those few animals who have defeated the tides of time by staying alive through chains of thick and thins millions and millions of years without accepting any evolutionary change in their basic body structure and habitat and every thing and features concerned with them.

Horseshoe crab

These are marine and brackish water arthropods of the family Limulidae, suborder Xiphosurida, and order Xiphosura. Their popular name is a misnomer, as they are not true crabs, which are crustaceans.Horseshoe crabs live primarily in and around shallow coastal waters on soft sandy or muddy bottoms. They tend to spawn in the intertidal zone at spring high tides. They are commonly eaten in Asia, and used as fishing bait, in fertilizer and in science (especially Limulus amebocyte lysate). In recent years, population declines have occurred as a consequence of coastal habitat destruction and overharvesting. The entire body of the horseshoe crab is protected by a hard carapace. It has two compound lateral eyes, each composed of about 1,000 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.

Coelacanth

The coelacanths constitute a now-rare order of fish that includes two extant species in the genus Latimeria: the West Indian Ocean coelacanth (Latimeria chalumnae) primarily found near the Comoro Islands off the east coast of Africa and the Indonesian coelacanth (Latimeria menadoensis). They follow the oldest-known living lineage of Sarcopterygii (lobe-finned fish and tetrapods), which means they are more closely related to lungfish and tetrapods than to ray-finned fish. They are found along the coastline of Indonesia and in the Indian Ocean. The West Indian Ocean coelacanth is a critically endangered species.

Coelacanths belong to the subclass Actinistia, a group of lobed-finned fish related to lungfish and certain extinct Devonian fish such as osteolepiforms, porolepiforms, rhizodonts, and Panderichthys. Coelacanths were thought to have become extinct in the Late Cretaceous, around 66 million years ago, but were rediscovered in 1938 off the coast of South Africa.The coelacanth was long considered a “living fossil” because scientists thought it was the sole remaining member of a taxon otherwise known only from fossils, with no close relations alive, and that it evolved into roughly its current form approximately 400 million years ago.However, several recent studies have shown that coelacanth body shapes are much more diverse than previously thought.

Nautilus

The nautilus is a pelagic marine mollusc of the cephalopod family Nautilidae, the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina.It comprises six living species in two genera, the type of which is the genus Nautilus. Though it more specifically refers to species Nautilus pompilius, the name chambered nautilus is also used for any of the Nautilidae. All are protected under Nautilidae, both extant and extinct, are characterized by involute or more or less convolute shells that are generally smooth, with compressed or depressed whorl sections, straight to sinuous sutures, and a tubular, generally central siphuncle. Having survived relatively unchanged for hundreds of millions of years, nautiluses represent the only living members of the subclass nautiloidea, and are often considered “living fossils”.

The word nautilus is derived from the Greek ναυτίλος nautílos and originally referred to the paper nautiluses of the genus Argonauta, which are actually octopuses. The word nautílos literally means “sailor”, as paper nautiluses were thought to use two of their arms as sails.