Lamellibrachia luymesi van der Land & Norrevang, 1975, is a large tube-dwelling marine worm. The only known specimen was collected in the Atlantic Ocean off Guyana at a depth of 500 m. A detailed description is given of the external morphology, the anatomy and the histology of this male specimen. Comparisons are made with the only known relative, Lamellibrachia barhami Webb , 1969, occurring off the Pacific Coast of the United States. The two species of Lamellibrachia form a group of their own, the Vestimentifera, which is quite distinct from all other worms. They range among the most interesting recent discoveries of deep sea animals. The body of the Vestimentifera consists of a muscular anterior part (vestimental region), provided with two lateral "wings", a very long tapering trunk and perhaps a small third region (opisthosoma). The anterior end carries two obturacula, which together form an operculum-like structure, and thousands of filiform tentacles (tentacular region). The regionation of the body cannot easily be compared with that of other animals. Perhaps the presence of two pairs of coelomoducts (the nephridia and the gonoducts) and an opisthosoma indicates that the ancestors of the Vestimentifera were segmented worms. When in the tube the vestimental wings are folded over the dorsal side, thus forming a cavity, which is for the greater part lined with glandular epithelium. The function of the unique vestimental region remains obscure. The most remarkable aspect of the anatomy is the complete lack of an intestine, at least in the adult. The question how these rather large animals can live without a gut remains unanswered. They share this unusual character with the Pogonophora, but these animals are extraordinarily thin. The Vestimentifera and the Pogonophora have several other characters in common, but there are also many important differences, so that they cannot be regarded as directly related groups. The authors consider the Vestimentifera as well as the Pogonophora separate classes of the phylum Annelida, A broadening of the concept Annelida is probably the best solution to the systematic problems raised by the discovery of these two groups.