These animals have a large body cavity, a closed circulatory system, a complex brain, a highly responsive nervous system and head with large & complex eyes. They are small; maximum length (excluding tentacles) is 10 cm. Their bodies are short and rounded at the posterior end. They have 8 short arms (each with 2-4 rows of suckers) and 2 long arms, which can be almost entirely withdrawn into the body. The shell, or "pen," is internal, slender, and very thin and delicate. These squid also have large, semi-circular fins on the middle of the sides of the body.