INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE MULTICELLULAR ANIMALS (EUMETAZOA)

SECTION TRIPLOBLASTIC (TRIPLOBLASTICA) OR BILATERAL (BILATERIA) ANIMALS

SUBSECTION LOPHOPHORATES (LOPHOPHORATA OR TENTACULATA)

PHYLUM BRACHIOPODA (LAMP SHELLS)

CLASS BRACHIOPODA

Brachiopods are marine animals adapted to normal salinity. Their body size typically ranges within a few centimeters, with the largest living species, Magellania venosa, reaching up to 8.4 cm.

Anatomy. The soft body of brachiopods is enclosed in a bivalve shell, where the ventral valve is generally larger than the dorsal one. Its open, rounded

margin corresponds to the anterior end of the body, while the closed, more pointed margin represents the posterior end (Fig. 413). The chemical composition of the shell varies between the classes Inarticulata and Articulata. In inarticulate brachiopods, the shell contains a high proportion of organic matter, with calcium and magnesium phosphates serving as the main Inorganic Components; in articulate brachiopods, organic matter accounts for only 2%, and calcium carbonate replaces phosphates. In inarticulate brachiopods, the Valves are held together at the posterior end solely by Muscles, whereas in articulate brachiopods, they are interlocked by specific processes on the posterior margin of the ventral valve that fit into sockets on the dorsal valve, forming a sturdy and inseparable hinge. This hinge permits only a slight gaping of the valves.

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Fig. 413. Brachiopods attached to the substrate by a pedicle (after Abrikosov)

Occasionally, brachiopods attach to the substrate directly via the ventral valve (e.g., Crania), but most commonly they do so using a specialized body outgrowth called a FOOT, or pedicle, which firmly anchors to rough substrates either across its entire basal surface or through ROOT-like branching extensions.

The soft body occupies the posterior third of the shell, with its anterior end bearing two arms fringed with short tentacles. In small species, these arms are simple and ring-shaped, whereas in larger ones, they are spirally coiled (Fig. 414) and possess a greater number of tentacles. The tentacles are covered with cilia; their beating currents drive Water between the tentacles. A deep, heavily ciliated groove runs along the outer surface of the arms, channeling small food particles down to the Base of the arms and into the Mouth. Ciliary movement also ensures a continuous renewal of water within the mantle cavity, which facilitates Respiration.

Fig. 414. Internal anatomy of a brachiopod (the dorsal valve and mantle have been removed) (after Abrikosov):

1 - right arm; 2 - dorsal end of the adductor muscles; 3, 4 - lobes and sectioned ducts of the digestive gland, respectively; 5 - Stomach;

6 - Heart; 7 - pedicle; 8 - nephridium; 9 - Blood vessel; 10 - pedicle Muscle; 11 - terminal thickening of the hindgut;

12 - ventral end of the adductor muscle; 13 - Esophagus; 14 - anterior wall of the soft body; 15 - left arm;

16 - ventral valve of the shell

Integument. The body of brachiopods is covered by a single-layered epithelium that extends onto the dorsal and ventral sides to form a double-layered fold known as the mantle, which lines the mantle cavity. Beneath the epithelium lies a thin layer of Connective Tissue, followed deeper by a ciliated peritoneal epithelium that encloses a spacious coelom. Extensions of the coelom penetrate between the two layers of the mantle, forming a rather complex network there, and also extend into the arms to keep the tentacles extended.

Musculature. Subcutaneous musculature is absent in brachiopods due to the presence of the shell. The Muscular System is differentiated into distinct bundles that connect the shell valves (in inarticulate forms), open and close them, and shift them relative to one another. The pedicle, protruding between the shell valves, possesses its own intrinsic musculature.

The Digestive System begins with a small, slit-like mouth located at the bottom of a circumoral depression, which is an expansion of the ciliated arm groove. The mouth leads into the esophagus, followed by The Stomach, into which the ducts of numerous digestive (hepatic) glands empty. In articulate brachiopods, the narrow hindgut ends blindly, whereas in inarticulate brachiopods, it opens to the outside via an anus on the right side of the body. Digestion AND ABSORPTION take place within the hepatic glands. The intestine is looped and suspended in the body cavity by mesenteries. Brachiopods feed on small planktonic Algae and invertebrates.

The excretory system is represented by one or two pairs of tubular coelomoducts. One end of the coelomoduct opens into the coelom via a funnel-shaped expansion, while the other, narrow end opens externally through a small pore.

The Circulatory system is open. The central organ is The Heart, which takes the form of an elongated muscular sac. A single aorta emerges from the heart, branching initially into two and subsequently into numerous Arteries that supply the mantle and gonadal sinuses, approach the coelomoducts, and extend into the arms and tentacles. Venous vessels are absent. The blood is colorless and lacks formed elements. Blood Circulation is driven by heart contractions and auxiliary pulsating dilations along the vessels, such as those near the coelomoducts.

The respiratory function is performed mainly by the arms, which, thanks to numerous thin-walled tentacles, provide a large surface area for gas exchange.

The Nervous system forms a circum-oesophageal ring with paired supra-oesophageal and unpaired sub-oesophageal ganglia. The supra-oesophageal ganglion innervates the arms, while the sub-oesophageal one supplies the rest of the body Organs.

Specialized Sense Organs are absent. Along the anterior margin of the mantle, some species feature pigmented areas that are sensitive to light.

Reproductive System. All brachiopods, with the exception of three Mediterranean hermaphroditic species of the genus Argyrotheca, are dioecious. Two pairs of Gonads are located within the coeloms of the dorsal and ventral mantle folds. They are racemose in shape; the Germ Cells are released into the coelom and subsequently discharged outward through coelomoducts.

Reproduction. Egg Fertilization takes place either within the mantle cavity of the female (subclass Articulata) or in the surrounding water (subclass Inarticulata). In the former case, the embryo develops inside the mantle cavity within the folds of the arm epithelium or within a specialized brood pouch formed at the base of the arms.

Cleavage is total, equal, and approaches a radial pattern; the mesoderm develops as a pair of pouch-like outgrowths of the entodermal gut, which subsequently pinch off to form two coelomic sacs. In articulate brachiopods, the egg hatches into a planktonic larva whose body consists of three distinct regions: the apical (cephalic), trunk, and pedicle regions. The apical region resembles an umbrella fringed with cilia, bearing an apical plate and four eyespots; the trunk region features two hanging Skin folds (the dorsal and ventral mantle folds); and the pedicle region appears as a short papilla (Fig. 415, a). The gut is blind-ending, lacking both mouth and anus. The larva does not feed. After 10–30 days, it settles on the bottom, attaching to the substrate by its pedicle. Its apical region undergoes regression, the mantle folds secrete a shell, and arm rudiments begin to sprout near the mouth, subsequently elongating and developing tentacles.

In inarticulate brachiopods, the egg hatches into a larva that already possesses a lophophore with tentacles and a bivalve shell (Fig. 415, b). It leads a planktonic, feeding existence for about a month before settling on the substrate and transforming into an adult brachiopod without undergoing major organ reorganization.

Brachiopods predominantly inhabit the marine shelf zone, with only about 30 species found at depths exceeding 2 km, and three species recorded down to 6 km. The vast majority of brachiopods securely attach to hard substrates, whereas only members of the family Lingulidae (subclass Inarticulata) lead a burrowing lifestyle. They inhabit the intertidal and shallow subtidal zones, constructing vertical mucus-lined burrows in sandy and muddy sediments.

Fig. 415. Brachiopod larvae: a — articulate; b — inarticulate (adapted from Ivanova-Kazas):

1 — eyespots; 2 — apical lobe; 3 — gut rudiment; 4 — muscles; 5 — pedicle rudiment; 6 — setae;

7 — mantle fold; 8 — shell; 9 — preoral lobe; 10 — pedicle; 11 — tentacles

Representatives of the extant genus Lingula date back to the Ordovician, meaning this Lineage has existed for approximately 450 million years.

Fossil brachiopods are of great practical importance as index fossils for determining the geological age of various strata of the Earth's surface during mineral prospecting.

The remains of phosphatic shells of Ordovician species belonging to the genus Obolus formed so-called obolus deposits, which are utilized for The production of phosphate fertilizers, for instance, in Estonia.