Can be thought of as a Phoronid with hard shells.
All marine, found from intertidial level to the deep sea. Only about 350 living species; more then 30000 found as fossils.
Resemble bivalvic molluscs, have a mantle and bivalve shell - but this similarity is superficial. Valves dorsoventral (lateral in molluscs). Dorsal and ventral valves are morphologically different. Ventral - pedicle valve, dorsal - brachial valve with lophophore. Shells are calcite or chitinophosphate (aragonite in molluscs).
from 1mm to 9cm in longest shell dimension. Sit attached to substrate, filter-feeding with lophophore.
U-shaped digestive system: mouth => oesophagus => stomach => intestine.
No specialized respiratory organs - gas exchange through lophophore and mantle. Open, very reduced circulatory system. Contracticle heart lies just above gut; after leaving dorsal vessel, blood moves in system of interconnected blood sinuses. Blood contains no oxygen-binding pigments, but haemoerythrin found in coelomic fluid (for nutrient distribution?)
Excretion by 1 or 2 pairs of metanephridia.
No asexual reproduction; sexes separate. Gametes develop from gonadal tissue and excreted to metacoel and out through nephridia. In most cases fertilisation external; some species known to brood embryos until larval stage and fertilize internally.
Deuterostomous development - radial cleavage, secondary mouth formation, enterocoely. Special free-swimming larval form - "lobate larva".
2 classes:
Articulata - shells attach by tooth and socket hinge, live attached by substrate. (about 300 species). Pedicle may be short with no muscle or muscular. A few species have no pedicle and attach by ventral valve.
Shell opened by deductor muscle, the hinge prevents large gap. Closed by adductor muscle.
Intestine ends blindly (secondary loss of anus).
Inarticulata - shells basically unattached and only held together by muscles. Live in soft substrate. Long, muscular pedicle used to pull animal down burrough. Some active, moving by setae and pedicles.
No deductor muscles. Gape by retracting the body, increasing pressure of coelomic fluid, forcing valves apart. Intestine ends with rectum and anus. Opens medially or to the right side of mantle cavity.