Conversely, inarticulate brachiopods have weak, untoothed hinges and a more complex system of vertical and oblique (diagonal) muscles used to keep the two valves aligned.

At their peak in the Paleozoic era, the brachiopods were among the most abundant filter-feeders and reef-builders, and occupied other ecological niches, including swimming in the jet-propulsion style of scallops.

A study in 2007 concluded the brachiopods were especially vulnerable to the Permian–Triassic extinction, as they built calcareous hard parts (made of calcium carbonate) and had low metabolic rates and weak respiratory systems.

One brachiopod species (Coptothyrus adamsi) may be a measure of environmental conditions around an oil terminal being built in Russia on the shore of the Sea of Japan.

Inarticulate brachiopods use a different opening mechanism, in which muscles reduce the length of the coelom (main body cavity) and make it bulge outwards, pushing the valves apart.

[16] In most species the edge of the mantle also bears movable bristles, often called chaetae or setae, that may help defend the animals and may act as sensors.

The function of these diverticula is uncertain and it is suggested that they may be storage chambers for chemicals such as glycogen, may secrete repellents to deter organisms that stick to the shell or may help in respiration.

[9] Experiments show that a brachiopod's oxygen consumption drops if petroleum jelly is smeared on the shell, clogging the diverticula.

[9] The method used by brachiopods is known as "upstream collecting", as food particles are captured as they enter the field of cilia that creates the feeding current.

Entoprocts use a similar-looking crown of tentacles, but it is solid and the flow runs from bases to tips, forming a "downstream collecting" system that catches food particles as they are about to exit.

[23] In a few articulate genera such as Neothyris and Anakinetica, the pedicles wither as the adults grow and finally lie loosely on the surface.

[16] A lingulid moves its body up and down the top two-thirds of the burrow, while the remaining third is occupied only by the pedicle, with a bulb on the end that builds a "concrete" anchor.

[16] The lophophore captures food particles, especially phytoplankton (tiny photosynthetic organisms), and deliver them to the mouth via the brachial grooves along the bases of the tentacles.

[26] Food passes through the mouth, muscular pharynx ("throat") and oesophagus ("gullet"),[9] all of which are lined with cilia and cells that secrete mucus and digestive enzymes.

[16] Ripe gametes float into the main coelom and then exit into the mantle cavity via the metanephridia, which open on either side of the mouth.

Both fossils and extant species have limitations that make it difficult to produce a comprehensive classification of brachiopods based on morphology.

It is now clear the brachiopods do not belong to the Deuterostomia (such as echinoderms and chordates) as was hypothesized earlier, but should be included in the broad group Protostomia, in a subgroup now called Lophotrochozoa.

Consequently, it has been suggested to include horseshoe worms in the Brachiopoda as a class named Phoronata (B.L.Cohen & Weydmann) in addition to the Craniata and Lingulata, within the subphylum Linguliformea.

Most species avoid locations with strong currents or waves, and typical sites include rocky overhangs, crevices and caves, steep slopes of continental shelves, and in deep ocean floors.

[2] Rhynchonelliforms, whose larvae consume only their yolks and settle and develop quickly, are often endemic to an area and form dense populations that can reach thousands per meter.

While brachiopods were abundant in warm, shallow seas during the Cretaceous period, most of their former niches are now occupied by bivalves, and most now live in cold and low-light conditions.

This may make the population of Coptothyrus adamsi useful as a measure of environmental conditions around an oil terminal being built in Russia on the shore of the Sea of Japan.

However, fossils of a new tommotiid, Eccentrotheca, showed an assembled mail coat that formed a tube, which would indicate a sessile animal rather than a creeping slug-like one.

Eccentrotheca's organophosphatic tube resembled that of phoronids,[55] sessile animals that feed by lophophores and are regarded either very close relatives or a sub-group of brachiopods.

[56] Paterimitra, another mostly assembled fossil found in 2008 and described in 2009, had two symmetrical plates at the bottom, like brachiopod valves but not fully enclosing the animal's body.

[57] At their peak in the Paleozoic,[58] the brachiopods were among the most abundant filter-feeders and reef-builders,[59] and occupied other ecological niches, including swimming in the jet-propulsion style of scallops.

[60] In 2007 Knoll and Bambach concluded that brachiopods were one of several groups that were most vulnerable to the Permian–Triassic extinction, as all had calcareous hard parts (made of calcium carbonate) and had low metabolic rates and weak respiratory systems.

On the other hand, warmer periods, such much of the Silurian, created smaller difference in temperatures, and all seas at the low to middle latitudes were colonized by the same few brachiopod species.

[72] The phoronids feed with a lophophore, burrow or encrust on surfaces, and build three-layered tubes made of polysaccharide, possibly chitin, mixed with particles with seabed material.

Traditionally they have been regarded as a separate phylum, but increasingly detailed molecular phylogeny studies between 1997 and 2000 have concluded that phoronids are a sub-group of brachiopods.