Underside and circulatory system of the Atlantic Horseshoe Crab (Limulus polyphemus)
The curious horseshoe crab (or king crab) is not actually a crab at all. It is an ancient member of the Chelicerata - more closely related to scorpions and spiders than any crustacean.
Note that the colors on the bottom illustrations only indicate a state of oxygenation and deoxygenation (red and blue, respectively), not the true color of the blood. Since horseshoe crabs utilize the copper-based hemocyanin to transport oxygen (as opposed to hemoglobin, in vertebrates), their blood is colorless or a light yellow when oxygenated, and a deep blue color when deoxygenated.
Of note - the mouth of the horseshoe crab is a jawless opening leading to a gizzard, between the legs on the underside of the body, and the book gills are located directly below them. The book gills, in addition to being used for blood gas exchange, are occasionally used for increased motility.
Top: Anatomy of underside and tail. From the John Reeves Collection, via scientificillustration. 1827.
Bottom: Circulatory system. Recherches sur l’Anatomie Limulus. M. Alphred Milne Edwards, 1873.