INVERTEBRATE ZOOLOGY IN THREE BOOKS - BOOK 3 - H.I. Shcherbak - 1997

PHYLUM BRACHIOPODA (BRACHIOPODA)

Brachiopods are exclusively marine benthic animals that lead a sessile lifestyle. Around 280 extant and over 10,000 fossil species have been described.

Externally, brachiopods resemble Mollusks, as their soft body is enclosed in a bivalve shell. Consequently, for a long time they were classified alongside mollusks and, by analogy with other mollusk classes (Gastropoda, Cephalopoda), they were named Brachiopoda (arm-footed); however, in their Anatomical Structure, they have nothing in common with mollusks.

Unlike mollusks, the shell Valves of brachiopods cover the soft body from the ventral and dorsal sides rather than laterally. The body occupies only one-third of the shell, while the remaining two-thirds are lined by a two-layered fold—the mantle—enclosing the mantle cavity. This cavity houses the filtering apparatus, which is formed by bodily outgrowths known as arms, typically covered with numerous tentacles.

Brachiopods are coelomate animals; their Muscular System includes individual Muscles of the body wall and various Organs, as well as specialized muscles, among which the shell-opener and closer muscles are the most developed.

The Digestive System is either complete or lacks an anal opening. The excretory organs are represented by one or two pairs of coelomoducts. There are no specialized respiratory organs, and their function is largely performed by the arms. The Circulatory system includes a Heart and a vascular network. The Nervous system is poorly developed, consisting of a circumesophageal nerve ring that innervates the entire body. Specialized Sense Organs are absent.

Most brachiopods are dioecious. Gonads are located within the coelomic cavities of the mantle, typically forming two pairs. Development occurs either directly or through metamorphosis.

The phylum Brachiopoda comprises a single Class of the same name, which includes two Major Groups of species distinguished as subclasses: Inarticulata (or Ecardines) and Articulata (or Testicardines).