Summary1. The study of the microfauna living in the interstitial water in marine or freshwater sand, the so‐called interstitial or mesopsammic fauna, has made important contributions to systematic zoology during the past four decades. Most invertebrate groups are represented, and many of the morphologically aberrant forms of animals discovered in this environment belong to quite new structural types. Among the discoveries are animal groups of high systematic rank, such as the orders Actinulida (Coelenterata), Gnathostomulida (Turbellaria), Mystacocarida (Crustacea) and Acochlidiacea (Mollusca).2. The coastal subsoil water is the environment of a special interstitial brackish‐water fauna, which has been studied intensively along the beaches of Europe. This zone may be regarded as a transitional area between the submerged marine sand and the continental subterranean waters with their phreatic freshwater fauna.3. One ecological factor of prime significance is that of space, which is dependent on the grain size distribution in a sediment, and which determines the upper size limit of organisms in a given interstitial environment. The granulometric characteristics of the sand affect the composition of the microbiocoenoses and their distribution within a beach area.4. Wind, waves and currents cause a continuous rearrangement of the particles in the surface layer of the sand. The interstitial system of the sand is therefore most usually a very dynamic environment with special biological conditions.5. Such physical factors as temperature and salinity vary greatly in many interstitial biotopes, particularly in the tidal zone. This implies that the interstitial organisms are physiologically adapted to endure both the seasonal variation and the often rapid changes that occur in connexion with ebb and flood. The littoral interstitial fauna is therefore eurythermal and euryhaline.6. The vertical distribution of the interstitial fauna in a sandy beach varies with the situation in relation to low‐ and high‐water marks. The vertical and also the horizontal distribution of the microfauna in a beach show a marked variation with the season of the year, which is usually manifested as a migration towards greater depth during the colder part of the year.7. It is in the interstitial fauna that we find the very smallest representatives of most of the invertebrate phyla. Body sizes vary from about 0.5 to a few millimetres, and only thread‐shaped organisms are longer than this. Protozoa and Metozoa in this environment have about the same dimensions. Examples of morphological regression in the interstitial fauna suggest that body lengths of about 0.5–1 mm. may be the low size limit for invertebrates.8. Certain shapes of body dominate in the interstitial fauna. Elongated forms are favoured; vermiformity is common and may occur in groups of animals in which such a body shape is unusual (the coelenterateHalammohydra, the opisthobranchPseudovermisand others). Another type is represented by the broad and flat forms.9. Morphological adaptations to the biological demands of the dynamic environment are found. These consist of (a) different kinds of reinforcement of the body wall, such as cuticular scales or spines (gastrotrichs, solenogastrids) or epidermal spicules (turbellarians, opisthobranchs), which are of importance as mechanical protection; (b) an often marked ability of contraction serves a similar purpose in organisms with thin body walls (ciliates, turbellarians, gastrotrichs and others); (c) adhesive organs are found in most species, and attachment is by adhesive glands or various kinds of gripping organs; (d) static sense organs are common in different groups of animals and the importance of such organs in a dynamic environment is obvious.10. As regards modes of nutrition, the following categories dominate: (a) predators, e.g. coelenterates, turbellarians, nematodes; (b) diatom‐ and epi‐growth feeders, which may be divided into browsers (archiannelids, crustaceans, molluscs), pump‐suckers (some turbellarians, gastrotrichs, nematodes,Psammodrilus), puncture‐suckers (tardigrades) and sand‐lickers (certain amphipods, cumaceans); (c) detritus‐feeders (some gastrotrichs, nematodes and archiannelids); (d) suspension‐feeders (sedentary forms:Monobryoxoon, a brachiopodGwyniaand the interstitial ascidians).11. The production of gametes is usually low; 1–10 ova per female at a time is normal. Adaptations for the maintenance of the populations of low‐producing species are: (a) spermatophores to ensure fertilization, e.g.Protodrilus, Microhedyle; (b) embryonic and larval development in cocoons, fixed to the substratum; (c) larval development with the suppression of a pelagic phase; (d) brood protection, e.g.Otohydra, Neril‐lidae; (e) considerable extension of the period of reproduction.12. The Ciliata are important in the sand microfauna, where they are represented by almost 90 genera. The ciliates of fine‐sand habitats have been found to be the best adapted to interstitial conditions (microporal ciliates).13. Coelenterates are represented by a rather small number of strongly aberrant forms. Among the Hydrozoa the genusHalammohydrais the best known. The genus forms a morphological series with transitions from ovoid to vermiform species, each adapted to definite interstitial environments, The peculiar bipolar genusSphenotrochusrepresents the Madreporaria.14. Of the two orders of gastrotrichs, the Macrodasyoidea, with itsca. 70 species, occurs exclusively in the interstitial fauna; the Chaetonotoidea are also common here, though this particular group has its main distribution in fresh waters. The macro‐dasyoid gastrotrichs are one of the groups characteristic of the sand microfauna, and provide possibilities for us to study different kinds of adaptation to the conditions of the environment.15. The turbellarians comprise a large and varied group found in practically all types of sand biotopes. Those best adapted to the interstitial conditions are the Kaly‐ptorhynchia, and the Otoplanidae family. The order Gnathostomulida is one of the newly discovered aberrant groups. Members of this group are reminiscent of the familiar turbellarians, but differ in important structural features (cuticular pharyngeal jaws, polygonal epidermal epithelium).16. The Nematoda in the sand fauna are rich in species and occur not infrequently in very large numbers. The distribution of the species in different sand biotopes has proved to be dependent on such ecological factors as grain size and supply of food.17. The undoubtedly polyphyletic Archiannelida form one of the more characteristic groups of the interstitial fauna. Sixty or so interstitial species are known, but only few from other biotopes.18. Ostracoda, Mystacocarida, Copepoda and Isopoda are the most important groups of crustaceans in marine sand. Elongated body forms or elongated shell types (Ostracoda) are common in the interstitial species.The order Mystacocarida, discovered in 1943, is known exclusively from the interstitial fauna, mainly in coastal subsoil water. These very small crustaceans, related to the copepods, have played an important role in phylogenetic discussions.19. The best represented of the mollusc groups are the Opisthobranchia, with the order Acochlidiacea, containing ten species in the size range of 0.8–3 mm., and more or less vermiform. Other typical sand microforms are the generaRhodope, PseudovermisandPhilinoglossa.The groups of molluscs in the sand microfauna to which least attention has been paid are the solenogastrids, which are represented in the so‐calledAmphioxus‐sand by species about a millimetre long.20. The remarkableMonobryozoon ambulansis the only bryozoan in this environment. It has a restricted locomotive ability and may be regarded as semi‐sessile.21. The Echinodermata are represented by a few synaptids a couple of millimetres long, e.g.Leptosynapta minuta, which by shape of body, method of locomotion, adhesive ability, static organs, etc., is very well adapted to life in the interstitial environment.22. Recently discovered interstitial Ascidiacea are ree they belong to various families. Some of the interstitial species have a certain locomotive ability, due to muscle activity.