INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE MULTICELLULAR ANIMALS (EUMETAZOA)

DIVISION TRIPLOBLASTIC (TRIPLOBLASTICA) OR BILATERAL (BILATERIA) ANIMALS

SUBDIVISION LOPHOPHORATA (TENTACULATA)

PHYLA BRYOZOA (ECTOPROCTA)

CLASS GYMNOLAEMATA

Gymnolaemates are predominantly marine animals, with representatives of only three genera inhabiting brackish and freshwaters. Their colonies, much like those of the previous Class, exhibit a variety of forms (Fig. 406, c); however, unlike phylactolaemates, their colonies are polymorphic, meaning they consist of zooids that differ in both shape and function.

Anatomy. The foundation of the colonies is formed by ordinary feeding zooids, or autozooids, which have an egg-shaped, cylindrical, tubular, or chambered

Structure. Alongside them, the colonies contain modified zooids: avicularia, vibracula, and cenozooids, which perform various Functions (Fig. 411).

Avicularia are zooids with a heavily reduced polypide, functioning to protect the colony from predators. Externally, avicularia resemble a bird's HEAD (hence the name, from Lat. avis - bird). Vibracula feature a special long, movable appendage that performs vibrating movements driven by specialized Muscles, warding off predators from the colony and sweeping foreign particles from its surface. Cenozooids are tubular or plate-like individuals that perform mechanical functions: supportive (stolons) or attachment structures (ROOT-like tubes or plates used by the colony to anchor to the substrate). In addition to actively defensive individuals, many species possess passive defense structures—various outgrowths of the outer wall, such as spines, prickles, etc. In some forms, these completely encase the colony, making it impregnable.

Fig. 411. Modified zooids of gymnolaemates

(after Abrikosov): a, b - structural diagrams of an avicularium and a vibraculum, respectively; c - part of a Caberea ellisii colony with vibracula:

1 - stalk; 2, 3 - muscles opening and closing the Mandible; 4 - finger-like process with sensory setae; 5 - Muscle attachment site on the mandible; 6 - mandible; 7 - flagellum; 8 - muscles moving it; 9 — cystid

Gymnolaemate bryozoans possess a ring-shaped lophophore with a small number of tentacles—ranging from 8 to 18 (with the exception of Flustra hispida, which has 30 tentacles). An epistome is absent. The body walls of most gymnolaemates are externally covered by a chitinous cuticle impregnated with calcium carbonate and traces of magnesium salts. The body wall musculature is generally less developed than in phylactolaemates, with retractor muscles being the most powerful. Coelomoducts are lacking in gymnolaemates; the excretory function is performed by amoebocytes of the coelomic fluid (Fig. 408).

Reproduction. Eggs develop within the maternal Organism, in the coelom, or in oeciis (ovicells). Different species produce various microscopic larvae that differ in appearance, integument structure, and the degree of gut development. The cyphonautes larva possesses the most complex structure (Fig. 412). Its body is enclosed in a bivalve shell; a tuft of cilia is located at the apical pole; and a well-developed V-shaped gut is present. The lower pole bears a sucker for attaching to the substrate. The period of active larval life ranges from a few hours in some species to anywhere from a month to a year in others. Subsequently, the larva settles to the bottom and attaches to the substrate. Metamorphosis begins: at the pole opposite the attachment site, unlike in phylactolaemates, a single individual—the ancestrula, which founds the future colony—is laid down. Most larval Organs degenerate in the process, including the gut with its endodermal section, while the newly formed gut consists exclusively of ectoderm. Colony formation is driven by asexual reproduction via budding.

Fig. 412. Cyphonautes larva of the gymnolaemate bryozoan Farella repens (after Ivanova-Kazas)

Processes of degeneration and regeneration play a significant role in the life of gymnolaemate colonies. Each colony, within its older regions, contains so-called brown bodies—degenerate polypides. Typically, a new polypide forms in place of a dead one.

Gymnolaemates are distributed across all seas, particularly in tropical and subtropical zones. They inhabit a wide depth range—from the intertidal zone

down to depths of 6 km, preferring hard substrates such as rocks, stones, and shell rubble, while only a few species settle on soft sandy or silty bottoms. They are frequently found on Algae, the tubes of sessile polychaetes, and mollusk shells. Their colonies vary greatly in form, much like those of phylactolaemates, but even more so due to zooid polymorphism and their bizarre spatial arrangements.

Gymnolaemates, particularly forms lacking calcareous walls, play a notable role in marine ecosystems as a food source for fish, birds, sea urchins, sea cucumbers, and polychaetes. Bryozoan larvae serve as a particularly valuable food source for juvenile fish.

The Practical significance of bryozoans stems from their tendency to foul ship hulls and various underwater structures, especially hydraulic engineering facilities such as marine Water intakes. The species Plumatella fungosa poses a particular threat to water supply systems, including those in Ukraine. After colony die-off, their remains and statoblasts clog water distribution networks.

Bryozoans are one of the animal groups that have consistently contributed to reef building. Paleozoic bryozoans were the primary builders of bryozoan reefs alongside other reef-building organisms. Bryozoan reefs are known in many PARTS OF THE globe. In Ukraine, these include the reefs of the Kerch Peninsula, which are the exclusive result of bryozoan activity.

Along with foraminifers, naked bryozoans serve as characteristic index fossils for determining the age of various deposits in mineral exploration.