Thalassema thalassemum is an unsegmented, soft-bodied spoon worm. The main body, or trunk, is bilaterally symmetrical and reaches a length of up to 2-7 cm. The colour of the trunk varies between individuals and may be blue, grey, yellow, pink or purple. On the surface of the trunk are numerous papillae that are arranged in rings around the body, and which increase in number towards the posterior. Along the middle of the ventral surface of the body there is sometimes a single white line. The mouth is positioned at the base of the proboscis and behind this lies a pair of chaetae. The proboscis is coloured peach or cream, can extend up to 10-20 cm and may appear to have a ventral channel when its lateral margins roll inwards during feeding.Thalassema thalassemum is the smallest of the Echiurans. Its main body is capable of contracting to 2-3 cm. It is a deposit feeder, using its proboscis to secrete a mucus sheet that traps detritus. Thalassema thalassemum is gonochoristic, although there is no sexual dimorphism, and reproduces by external fertilization in the summer. A free-swimming planktonic larva stage develops from fertilized eggs before developing into the adult form. Thalassema thalassemum is mainly nocturnal.