There's a new sea spider in town

While there have been very abundant arthropods already several tens of millions of years before the first preserved sea spiders, almost all of them, for example the trilobites, were very unlike any present arthropods.

Sea spiders seem to be the first among arthropods who have reached a local optimum in body shape and structure, so they have not changed significantly after that, while also surviving various extinction events that have wiped out other groups.

Despite the name, terrestrial spiders are only very distant relatives of the sea spiders, and they have appeared only about one hundred and fifty million years later than the sea spiders. Terrestrial spiders are much more closely related to scorpions, than to sea spiders.

The body usually looks like a narrow twig that connects the big and long legs. This is why one of their alternative scientific names was "Pantopoda", which means "all-legs" in Ancient Greek.

Because there is not enough space in their body, many of their internal organs extend into the legs.