It is impossible to give a brief definition of a group that is so variable and incompletely known as the family Eulimidae. A family is a unit based on a number of genera which are more related to each other than to other genera and every new genus tends to strain the limits.

Shell. Usually present. Colourless or brownish yellowish with brownish or yellowish markings. Often there are one or several scars from earlier positions of the outer lip (similar scars may also be found in Aclididae and Rissoinidae). The shape of the shell is most variable. Siphonal canal absent. Larval shell. Brownish or colourless. In species with planktotrophic development it consists of 2.5-4 whorls and is rather slender. There is no sculpture except in a few species which have extremely faint axial lines. It does not show any sinusigera characteristics. Operculum. An operculum is present in all species with a solid shell, but is often lacking in species which are constantly attached to the host and have an inflated or less solid shell. Sometimes it has pegs, folds or other reinforcements.
The shell of most primitive eulimids is straight, conical, with flat whorls, a polished surface and a high spire. Many species have a more or less coloured shell, marked with brownish bands or spots on a colourless or yellowish background. These colour patterns are usually specific for the species, but fade in empty shells. I have, however, seen them in Eocene specimens.
Presence or absence of colour has sometimes been used to distinguish genera (Laseron, 1955), but I have observed several cases where species with coloured and colourless shells belong to the same genus, judging from anatomical characters. The shell is usually rather solid, more so than in most mesogastropods of comparable size and shape. The suture is very shallow and marked by a less transparent spiral band which constitutes that part of the whorl which is in contact with the preceding whorl. In many species the suture is so indistinct, that the lower part of the spiral band is more conspicuous than the real suture. Bartsch (1917) used the term “false suture” for this line and I have adopted his use.
In most eulimids the surface of the shell looks smooth at the first glance, but when examined with a stereomicroscope and good illumination there can often be seen extremely fine spiral and/or axial striae. These are especially distinct when the light is reflected by the shell. This is not a real sculpture. SEM examination of some species with such a striation, proved that the surface was completely smooth, even at high magnification. Therefore I suppose that this striation is a refractive phenomenon, caused by the crystalline structure of the calcium carbonate. It is, however, a good taxonomical characteristic, on the species level.
In some eulimids, especially Niso, but also scattered among the slender species of other genera, there is a sculpture of regularly spaced, sharp, distinct, raised axial lines. These lines run almost straight, from suture to suture. They are never present in species with inflated shells, and they should not be confused with incremental lines, which usually run parallel to the outer lip. In some species there is also a normal sculpture.
Almost all eulimids have scars from earlier positions of the outer lip. These are formed by the growth pattern typical for eulimids: they grow rapidly 0.3-1 whorl and then they stay at that size for a considerable time. During this standstill in growth, the outer lip is thickened and when it starts growing again, there is left a scar marking the position and the shape of the old lip. These scars appear very regularly in some species, in others the intervals are variable. In Melanella martini (A. Adams, 1855) some specimens have the scars in a perfect line, exactly one whorl from each other, while others have them scattered .
In some species with strongly-expanded apertures, e.g. Oceanida and Auriculigerina, these scars are very strong and may form varices or processes. One detail of taxonomic importance in many genera is the profile of the outer lip (seen from the side). In some species it is projecting (in relation to the part immediately below) at the suture, in others it is retracted and in some more or less perpendicular.
Two genera have an umbilicus, Niso and Microstilifer. The umbilicus in Niso is broad and deep and penetrates the shell up to the larval shell in many species. These species also have a strong basal keel. In other species of this genus the umbilicus is more narrow and the base rounded, and some lack it completely. In those species which anatomically may be regarded as more modified, the shell is usually less solid and more inflated. When scars are present, they usually represent a change in sex. Many of the odd genera such as Bacula, Concavibalcis, Amamibalcis etc. are still known from empty shells only, and it is not possible to say to what extent the oddness of the shells corresponds with deviations in the anatomy.

Tentacles. Usually present- They are round, flat, or are fused to form a fold. Sometimes they are lacking. Eyes are usually present and situated basally, under the skin in the centre of each tentacle. Radula. Present in Hemiliostraca, Niso, Eulimostraca, Eulima and some other genera. Ptenoglossate. Proboscis. Present in all except a few of the most highly reduced endoparasites. Acrembolic. Alimentary canal. Salivary glands present in some species. Oesophagus usually passing through the nerve ring anteroventrally in the body cavity. Stomach present in Eulima, but usually the oesophagus is gradually transformed into the midgut gland. Rectum often present. Pallial oviduct. Open. Penis. Present except in the most highly modified endoparasites. Seminal groove open. Foot. Usually present, often with flaps which may cover the base of the shell. Propodium (mentum) well developed.

Way of life. Always parasitic, more or less permanently attached to the host (with two exceptions echinoderms), by the snout or proboscis.
I have earlier used the name Eulimidae to denote these gastropods without discussing whether they should be regarded as a family or a higher taxon. Previous authors have distinguished between a number of taxa, a list of which is given below. These range from subfamily to suborder. As I will show later, these groups can all be derived from the basic eulimid organization shown by Eulima, Niso, and Melanella. Therefore I have preferred to keep them in one family, Eulimidae. I have not made any attempts to divide Eulimidae into subfamilies.

Diagnosis. Shell tall, slender, with color pattern of brown spiral bands; whorls flat-sided, aperture elongate; radula ptenoglossate.
Biology: Waren (1984:45) noted that the European Eulima bilineata is parasitic on ophiuroids. However, there are no records of hosts for eastern Pacific species.
Remarks. The tall shell with flat-sided whorls, elongate aperture and color pattern of brown lines characterize this genus. The genus was also discussed by Bouchet and Warén (1986:318) and Warén (1992:179).

Eulimids are recognized by their smooth, polished, white, high-spired or globose shells, and by their association with echinoderms. Unfortunately, there is no agreement as to the number of families to which these mollusks should be assigned, or the generic names which should be applied. Keen (1971) recognizes a single family, the Eulimidae; Abbott (1974) distinguishes the Melanellidae (= Eulimidae) with high spired, nonumbilicate shells, the Stiliferidae with globose shells parasitic on starfish and sea urchins, and the Aclididae with high-spired umbilicate shells; Ivanov (1952) and Gooding and Liitzen (1973) suggest that more than two families should be recognized. There is also argument as to whether Balcis, Melanella, or Eulima should be used for the high-spired white shells associated with holothurians and whether the small, fragile, dorso-ventrally compressed shells should be referred to as Subularia, Leiostraca, Eulima or Strombiformis (see Dell, 1956; Keen, 1971; Abbott, 1974). In this discussion I distinguish five genera in one family (adapted from Keen, 1971):
Eulimids exhibit varying degrees of mutualism with echinoderms: some are appar¬ently migratory, crawling freely over the surface of holothurians or among the spines of echinoids; some are attached to the host by a short proboscis; others are embedded in the epidermis of a sea cucumber or starfish, or live in the coelom. The anatomy is modified in association with their mode of life. Most apparently lack a radula and jaws, feeding by means of the pharynx which is modified as a buccal pump. Some are hermaphroditic, either protandrous or consecutive; others are dioecious. In Hawaii pelagic veliger larvae associated with 11 species have been found in the plankton; the protoconch is shaped like a teardrop and the velum is bilobed (J. B. Taylor, 1975).
Both shells and soft parts reflect the habits of the animals: those which are free-living (Balcis kanaka) have rather transparent shells and brightly colored animals; those which are attached to sea cucumbers (B. aciculata and Mucronalia nitidula) have solid, porcelaneous shells and the animals are white or decorated with yellow on the tentacles and foot. The endoparasites (Stilifer linckiae) that live in the body wall of starfish have thin, fragile shells which are covered by a large pseudopallium.