Centipede looking ocean critter. Anywhere from 3inches to a foot long. What is it? Purplish/blue!?

Question

Oregon coast. Some dead, some alive.

Answer 1

Perhaps it was a Horseshoe Crab.  Horseshoe crabs are distant relatives of spiders and are probably descended from the ancient eurypterids (sea scorpions).  They evolved in the shallow seas of the Paleozoic Era (540-248 million years ago) with other primitive arthropods like the trilobites. Horseshoe crabs are one of the oldest classes of marine arthropods, and are often referred to as living fossils, as they have changed little in the last 350 to 500 million years.

They spend the winters on the continental shelf and emerge at the shoreline in late spring to spawn, with the males arriving first. The smaller male grabs on to the back of a female with a "boxing glove" like structure on his front claws, often holding on for months at a time. After the female has laid a batch of eggs in a nest at a depth of 15-20 cm in the sand, the male fertilizes them with his sperm. Egg quantity is dependent on female body size and ranges from 15,000-64,000 eggs per female.[1]