While T. navalis looks like a brown worm on the outside, it is actually a bivalve. Its head is covered with two white, tri-lobed shells used to bore into wood. The shells are up to 2 cm long and have concentric ridges. Inside the shell is a hook-like process called a styloid apophysis. The foot is also at the anterior end. At the posterior end are two siphons: incurrent and excurrent. The former is used for respiration and feeding while the latter is where waste and sperm or larvae exit. Paddle-like pallets act as a lid to cover the siphons when not in use. Naval shipworms are about 20 cm in length but can range from 1.5 to 58 cm. They are 1 cm in diameter. Calcareous coverings are secreted from their mantles that coat the burrows they make. Male and female adults cannot be distinguished externally.
Range length: 1.5 to 58 cm.
Average length: 20 cm.
Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry
Sexual Dimorphism: sexes alike