REFERENCE NOTES FOR LECTURES IN ZOOLOGY

Module 4. MOLLUSCS, CHAETOGNATHS, PHORONIDS, BRYOZOANS, BRACHIOPODS, POGONOPHORANS, HEMICHORDATES, ECHINODERMS

Content Module 4.1. Molluscs, Chaetognaths, Phoronids, Bryozoans, Brachiopods, Pogonophorans

Topic 4.1.5. Phylum Brachiopoda (Brachiopods)

General characteristics of the phylum. Origin, evolution, paleontological data, modern diversity, geographical distribution, and their role in nature and human life. Overview of the main taxonomic groups important for human life and the functioning of natural ecosystems. Class Brachiopoda.

Exclusively marine benthic animals with a sessile lifestyle. Approximately 280 extant and over 10,000 extinct species. The soft body is enclosed in a bivalve shell. The shell Valves cover the body not laterally, but from the ventral and dorsal sides. The body occupies one-third of the shell, while the remaining two-thirds are lined by a two-layered

fold—the mantle—which defines the mantle cavity containing the filtering apparatus formed by body outgrowths (brachia) covered with numerous tentacles. Coelomate animals; The Muscular System includes individual Muscles of the body wall and various Organs, as well as specialized muscles, among which the adductor and diductor muscles that open and close the shell are the most developed. The Digestive System is complete or the anus is absent. Excretory organs are represented by one or two pairs of coelomoducts. Specialized respiratory organs are lacking, and their function is secondarily performed by the brachia. The Circulatory system consists of a Heart and a system of vessels. The Nervous system is poorly developed, featuring a circumesophageal nerve ring that innervates the entire body. Specialized Sense Organs are absent. Most species are dioecious. Gonads are located in the coelomic cavities of the mantle, in two pairs. Development occurs either with metamorphosis or directly.

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

Class Brachiopoda.

Inhabitants of normal-salinity marine environments. Sizes range in centimeters; the largest extant species, Magellania venosa, reaches 8.4 cm. The entire soft body is enclosed within the shell, with the ventral valve larger than the dorsal one. The open rounded margin of the shell represents the anterior end of the body, while the closed, more pointed margin represents the posterior end. The chemical composition of the shell varies between the Representatives of the subclasses Inarticulata (high organic content, with the inorganic portion composed of calcium and magnesium phosphates) and Articulata (organic matter constitutes only 2%, with the remainder being calcium carbonate). The shell surface is rarely smooth; it typically exhibits concentric ornamentation, including well-defined growth lines, as well as radial ornamentation in the form of folds, Ribs, and occasionally spines. At the posterior end, the shell valves are joined in inarticulates by means of muscles, whereas in articulates they are also joined by processes of the posterior margin of the ventral valve that fit into depressions on the posterior margin of the dorsal valve, forming a hinge—a sturdy articulation that cannot be disarticulated. Occasionally, brachiopods attach to the substrate via the ventral valve (Crania), but most commonly they attach by means of a specialized body outgrowth (the pedicle or stalk). The pedicle emerges from the shell either between the valves or through an opening in the ventral valve. Externally, it is covered by a thick horn-like cuticle. The pedicle firmly attaches to a rough substrate across its entire expanded posterior terminal pad or via ROOT-like branching extensions. At the anterior end of the body lie two brachia, which vary in length and shape among different species and are covered with varying numbers of tentacles. The tentacles are ciliated, and their beating currents drive Water between the tentacles. A deep ciliated groove runs along the outer side of the brachia, transporting small food particles down to the Base of the brachia into the Mouth. The Ciliary movement ensures a continuous exchange of water within the mantle cavity (facilitating normal Respiration).