The _Leptocephali_ proper are small, narrow, elongate, more or less band-shaped fishes, pellucid in a fresh state, but assuming a white colour when preserved in spirits, resembling a tapeworm, being quite as soft and flexible. The skeleton is entirely cartilaginous, or slight ossifications are only now and then visible, especially towards the end of the vertebral column. The latter is replaced by a chorda dorsalis which, in many specimens, is found to be divided into numerous segments. Neural arches are sometimes present in their rudimentary condition. The anterior end of the chorda passes into the cartilaginous base of the skull, the connection not being by means of joint and ligaments. Hæmal arches are found on the caudal portion. Ribs none. The skull, like the vertebral column, is nearly entirely cartilaginous. The basisphenoid, frontal, and jaw-bones are the first which may be distinguished, and the mandible has generally ossifications. The muscles are generally not attached to the chorda, which is surrounded by a thick gelatinous mass, separating the lateral sets of muscles from each other. These muscles are attached to the external integument, each forming a thin flat angular band, the angle being directed forwards. However, specimens are frequently found in which the muscles are more developed, evidently at the expense of the gelatinous matter, which is diminished in quantity. They are attached to the chorda, and the entire fish has a more cylindrical form of the body (_Helmichthys_). The nervous, circulatory, and respiratory organs are well developed. In those with a sub-cylindrical body the blood is red, in those with a flat body the blood-corpuscles show but rarely a faint coloration. There are four branchial arches, and in some (_Tilurus_) pseudobrauchiæ have been found. The gill-openings are more or less narrow. The nostrils are double on each side, and the posterior is close to the eye. The stomach has a large blind sac, and in _Leptocephalus_ two lateral cæca. The intestine is straight, running close to the abdominal profile, with a small appendix directed forward and a larger one directed backwards. The vent is nearly always very small, and, in preserved examples at least, cannot always be discovered. Its position is variable, even in examples entirely similar in other points. Air-bladder none. No trace of generative organs. The vertical fins, when present, are confluent, with more or less conspicuous traces of rays; sometimes they are merely a fold of the skin, without any rays. Pectoral fins sometimes present, sometimes rudimentary, sometimes entirely absent. Ventrals none. Most examples have series of round black dots along each side of the abdominal profile, along the lateral line, and sometimes along the dorsal fin. They remind us of the luminous organs of many _Scopelidæ_, _Stomiatidæ_, and other pelagic fishes, but are composed entirely of pigmentary cells.