Brachiopods are solitary marine bivalve coelomate invertebrates, which are symmetrical as related to axial section, normal to the plane of valve closing. The shell is calcareous or chitino-phosphatic. It is attached to substrate with a muscular pedicle or by secondary cementation. Both pedal and brachial valves of shell are layered inside with mantle appendages (lobes) of body-wall. The epistome appears as a brachial fold in front of the mouth (lophophore), situated between the mantle lobes and armed with filamentous tentacles. One or two pairs of metanephridia also serve as coelomoducts. All brachiopods are dioecious.

This short description of the phylum, accepted both by zoologists and paleontologists, can be complemented with the following details.

Since the brachiopod systematics is based chiefly on fossil material, identification of their families, genera and species relies mainly on the differences in the structure of shell and brachial skeleton (loop), supporting the lophophore — a respiratory and feeding (suspension sedimenting) apparatus. The differences include: absence or presence of narrow canals, normal to the shell surface, or of their ends (pores). This feature is easily discerned in transparent young specimens. In adults, pores can be seen in a fracture or at the inner surface of the shell, mostly near the margin of valves out of the fields for muscle and gonad attachment, which is usually covered with compact homogenous layer.

Unlike bivalves, having right and left valves, brachiopod shell valves are named pedal (ventral) and brachial (dorsal) ones. On the basis of newly obtained data on embryonal development it was supposed that both brachiopod shell valves belong to the dorsal side by their origin (Nielsen, 1991; Malakhov, 1995). Therefore, “brachial” and “pedal” are the most proper names for brachiopod valves. Pedal valve has an aperture (foramen), through which a pedicle goes out. Inside shell a pedicle is attached to the surface of a pedal valve with a muscle. Lophophore and skeletal structures, supporting it, are located in a brachial valve.

The hinge, insuring tight junction of the two valves, even in dead and fossilized specimens, may have or lack the cardinal process, situated at the middle of the shell posterior. In addition, it bears hinge plates, connecting crural bases to the middle septum and to margins of hinge-sockets. Two symmetrical teeth (dents) of the pedal valve are in a junction with the walls of hinge-sockets of the brachial valve, so tight that the valves cannot be disjoined without breaking the hinge.

The bases of the brachial skeleton (crura) may provide the only support for the lophophore(as in representatives of the order Rhynchonellida) or grow towards the shell anterior to form a loop, with its anterior part bridged with a transverse beam(as in representatives of the order Terebratulida). In some terebratulid species (e.g. those of the genusTerebratulina), the inner crural appendages, directed towards each other, knit to form, together with the crura and transverse beam, a closed ring. Long-looped brachial skeletons (as in families Laqueidae, Tythothyrididae, Terebrataliidae) are developing at the top of the median septum and knit with crura (by means of descending branches) only at the later stages of ontogenesis. In advanced forms such complicated skeleton is gradually losing its original connection with median septum during the growth and development of a specimen (connecting bands resolve), and forming a very long loop free of connection (as in Coptothyris adamsi). A set of genera in families of long-loop terebratulids usually makes more or less full row of ontogenetic stages, which is seen well as different levels of brachidium development. In the Sea of Japan, two genera, Terebratalia and Coptothyris, make up a fragment of such a series in the family Terebrataliidae. In the Bering and Okhotsk Seas an analogous series in the family Tythothyrididae is presented by a regressive series of three genera, Diestothyris, Tythothyris, and Simplicithyris (Zezina, 1979). Lophophores can have the same morphology with different brachial skeletons: in short-looped Terebratulina and in long-looped forms Laqueus, Diestothyris, Terebratalia, Coptothyris complicated three-lobed lophophores (with two loop-shaped side lobes and one spiral-shaped middle lobe) can be differentiated only by morphological features of skeletons and by the presence or absence of star-shaped carbonate spicules in soft tissues.

In feeding animals, filamentous tentacles of the lophophore, bearing cilia on the lateral and frontal sides, form a mobile wall. The wall blows the water from the inhalant to exhalant chamber of the mantle cavity, precipitating food particles (mostly flake-like remnants of plankton, bacteria and coagulated colloidal particles) and carrying them to the feeding groove, which takes them further to the mouth. Hard and extremely large particles initiate rejection reaction: one or several tentacles bend to let undesirable particles through the wall to the exhalant chamber. When the concentration of undesirable particles is extremely high (in particular, at mineral suspension and coarse plankton of high density), the valves close, and the animal stops feeding. Reduced hind gut and lack of anus is a distinction of articulate brachiopods (Testicardines). Undigested food remnants concentrated in the anterior part of the digestive tract are expelled through the mouth by periodical valve closures. The same mechanism is used for sexual products to be expelled. Spawning is studied in detail only for Hemithyris psittacea in the White Sea. Females of some Mediterranean, Antarctic, and high boreal species (the latter presented by the cancellothyroid Cnismatocentrum parvum from the Sea of Okhotsk) are known to nurse the embryos and larvae in the mantle camera (Malakhov, 1976). It is supposed that breeding of brachiopods takes place in spring and in autumn. Larvae of articulate brachiopods do not feed and soon settle near their parents, sometimes just on the surface of nearby adults to form crowds which can consist of different species. It is a very rare occasion to find larvae of articulate brachiopods in plankton.

Sometimes zoologists consider brachiopods as a taxon of higher rank than the Phylum (Beklemishev W.N., 1964, 1969; Beklemishev C.W., 1979), in particular, as subdivision Brachiopoda of division Bilateria (of the same rank as subdivision Deuterostomia). In recent times the unity of brachiopods as the phylum has been called in question (Starobogatov, 2000a, b), and inarticulate linguliform brachiopods with chitino-phosphate shells have been considered as Pleuropygia in the phylum Tentaculata (or Podaxonia) together with phoronids and bryozoans. But here inarticulate brachiopods are out of consideration because they are not found in the Russian waters of the Sea of Japan.

Nowadays most of zoologists, paleontologists, embryologists and biochemists-genetics (Kussakin, Drozdov, 1994; Malakhov, 1995; Williams et al., 1996, 1997; Cohen, Gowthrop, 1997; Peck et al., 1997; Cohen et al., 1998 a, 1998 b; Lueter, 2000; Ruppert et al., 2004) are of the opinion that the phylum Brachiopoda is a united one. In accordance with the system of high rank taxa (Williams et al., 1996), accepted for the new publication of “Treatise on Invertebrate Paleontology, Part H Brachiopoda (revised)”, there are three subphyla in the phylum Brachiopoda: Linguliformea, Craniaformea and Rhynchonelliformea. Among 8 classes of recent and fossil brachiopods (Lingulata, Paterinata, Craniata, Chileata, Obolellata, Kutorginata, Strophomenata, Rhynchonellata) only three ones are represented in the recent fauna (Lingulata, Craniata and Rhynchonellata) and only the last one is represented in the Russian waters of the Sea of Japan.

The idea of brachiopods as rare animals in recent marine fauna is not correct. 26 families, 116 genera and more than 300 species show that the diversity of recent forms is great. Fossil taxa are more numerous, but they are divided into many time periods and studied too locally. Diversity of fossil taxa is reflected in a big number of specialists studying them. The most paleontologists-brachiopodologists study Paleozoic taxa. Carbon and Perm are the time when brachiopods were in the most flourishing state. Recent forms represent the tops of different phylogenetic branches, which appear mostly in Cretaceous and Paleogene. Recent brachiopods are known from the northern seas of Europe, Asia and America up to the Antarctic. The most number of species of recent brachiopods is known (Zezina, 1976) in the tropical zone (31% of all recent species) and in the northern subtropical zone (32%). Geographic ranges of the species are very different, but mostly as large as the ranges of other groups of bottom invertebrates, and this fact disproves the idea of special big endemism in brachiopod fauna (it was the common opinion before the 70s of the 20th Century). In the Northern Hemisphere circumpolar arctic-boreal and North Pacific arctic-boreal species with very large geographic ranges are known. Deep-sea species (the deepest findings of living brachiopods are known at 6.8 km and of empty shells – at 7.6 km) have especially large ranges along meridian and in the Southern Ocean around the Antarctic Continent. These species are usually very eurybathic ones. The most recent species are confined to outer margin of shelf and upper part of slope where the belt of suspension-feeders is noted all around the World. In Primorye region we can see this belt well with three species of articulate brachiopods Laqueus vancouveriensis, Diestothyris frontalis, Terebratalia tisimana, and at the continental slope off Peter the Great Bay (at the depths 700–1600 m) Laqueus vancouveriensis forms spots of oligomixed communities where this species reaches density up to 3000 sp./m². Communities with brachiopods in leading position show location of natural deep-sea bottom biofilter, which together with shallow-water suspension-feeding communities forms a strong system of marine self-cleaning. Brachiopods can be good indicators to estimate the condition of this system, if we follow up changes in density and size structure of their populations.