Cephalotrhrix simula (Iwate 1953) is a ribbonworm (phylum Nemertea), a very long, thin unsegmented worm described from Japan. Nermerteans have a proboscis anterior to the mouth, which can be extruded through a pore anterior to the mouth, for the capture of prey. In Cephalothrix, the proboscis is unarmed, and is a simple tube, lacking a stylet. This worm stabs its prey with the proboscis, and paralyzes it with toxins. The body is creamy to yellowish brown, with orange on the snout, extending over head. The mouth has a large lip, extending into a sucker, The worm is up to 400 mm, and 1 to 2 mm in width. Ribbonworms move slowly by cilia, or more rapidly by muscular contractions. In its native Japan, it occurs in crustose red algae, and other compact red algae, and kelp holdfasts on tidal and barely subtidal rocky ledges. Cephalothrix simula, in Japan is known to concentrate very high concentrations of tetrodotoxin (TTX), one of the most lethal neurotoxins known. Very similar worms have recently been found in on the coast of Santa Barbara, California in 2009, San Diego Bay in 2010, the Netherlands in 2012, Mediterranean Spain in 2012, the Bay of Biscay in 2013, and Cornwall in 2018. Tetrodotoxin has been found in C. cf. simula from Japan, Spain, and England, and is assumed to occur elsewhere. These worms can occur in high concentrations in dense algal growths and oyster shells, and could pose a risk to people handling oysters, but the toxin has not been found in oyster meat. There is some evidence that the toxin is produced by bacterial symbionts of the ribbonworms. This worm could be transported as larvae in ballast water, or as adults in fouling, or with Pacific Oysters.