any member of the crustacean subclass Mystacocarida, a small group of primitive, free-living marine animals. Of the few species known, the first was discovered near Woods Hole, Mass., U.S., in 1943.

The shrimp's rather tubular body includes a long abdomen; thick, bristly antennules extend about two-thirds the length of the body. The largest species, Derocheilocaris galvarina, which attains lengths to 0.5 mm (about 0.02 inch), occurs on the Pacific coast of South America in the intertidal zone and on sandy bottoms in shallow waters. D. typicus occurs on the Atlantic coast of northeastern United States. D. remani is found on the coasts of Europe and Africa.