Physiology

Brachiopods are one of the few phyla to have lived in the world's oceans for the last 550 million years, since early Cambrian times. For much of that time they were the dominant shelled form in marine aquatic environments. Throughout all of this time their physiological structure has remained unchanged, and for some extant species their external anatomy is remarkably similar to early Cambrian forms (e.g., some inarticulated lingulids and some articulated rhynchonellides). They perform all of the basic physiological functions common to the vast majority of marine ectotherms, namely feeding, growth, reproduction, and metabolic and excretory processes. Their bodies are enclosed by two unequal shells, which are oriented dorsally and ventrally, as opposed to the lateral, equal shells in bivalve molluscs. The ventral valve in articulated species has a characteristic opening for the pedicle to pass through. In some articulated brachiopods the shells are traversed by many extensions of the mantle tissue called caeca (e.g., inWaltonia inconspicua, orTerebratulina retusa), while in others the shells form a continuous solid integument (e.g., rhynchonellides such asNotosaria nigricans). The former are the punctate brachiopods and the latter are impunctate species.