INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE MULTICELLULAR ANIMALS (EUMETAZOA)

SECTION TRIPLOBLASTICA (TRIPLOBLASTIC), OR BILATERIA (BILATERAL ANIMALS)

SUBSECTION LOPHOPHORATA (TENTACULATA)

PHYLA BRYOZOA (ECTOPROCTA)

Bryozoans (Bryozoa) (from the Greek bryon - moss, zoon - animal) are a large group of aquatic benthic animals of microscopic size (ranging from 0.3 to 5 mm) that form mono- and polymorphic colonies. In total, about 4,000 extant and 15,000 extinct species are known. Most bryozoans are predominantly inhabitants of the World Ocean, with significantly fewer species found in freshwater habitats.

The shape of bryozoan colonies is extremely diverse; most of them are arborescent or bushy, but There are also foliaceous forms, as well as those that creep along the substrate or encrust various objects (Fig. 406). Individual members of the colony, termed zooids, feature an anterior part—the polypide, bearing the Mouth and tentacles—and a posterior part—the cystid, enclosed within a cuticular cup. The tentacles are borne on a horseshoe-shaped or circular outgrowth known as the lophophore, in the center of which the mouth is situated (Figs. 407, 408).

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Fig. 406. General appearance of bryozoan colonies

(after Abrikosov): a - Fredencella sultana; b - Cristatella mucedo (Phylactolaemata); c - Retepora cellulosa (Gymnolaemata)

The body wall of the zooids consists of a chitinous cuticle. In polypides, it is thinner and more elastic, allowing the polypide to easily retract into the cystid. In marine bryozoans, the cuticle of the cystid is impregnated with calcium carbonate mixed with magnesium salts. An epidermis layer lies beneath the cuticle. The tentacles are covered with ciliated epithelium.

The musculature is represented by layers of circular and longitudinal Muscles in the body wall, as well as bundles of specialized muscles that ensure the rapid retraction of the polypide into the cystid.

The body cavity, or coelom, is divided by a septum of peritoneal epithelium into a small annular canal—which gives off branches into the tentacular apparatus—and the main coelomic cavity of the zooid. The peritoneal epithelium lines the inner surface of the body wall and surrounds the gut. The coelomic fluid contains free Cells, known as coelomocytes.

Digestive System. The gut is loop-shaped, with the anus located near the mouth, yet outside the lophophore.

Circulatory and respiratory systems, as well as the excretory system in many species, are absent. Respiration occurs across the general body surface, primarily through the thin walls of the tentacles.

The Nervous system consists of a single ganglion located between the mouth and the anus, from which nerves extend to the tentacles, body walls, and Internal Organs. The coordination of activity among all zooids in the colony is mediated by a colonial-wide nerve plexus. Special Sense Organs are absent.

Reproductive System. Most bryozoans are hermaphrodites. Testes and Ovaries develop within the peritoneal epithelial lining.

Reproduction. Bryozoans reproduce sexually, as well as asexually through budding, which drives colony growth. Development proceeds via metamorphosis.

The phylum Bryozoa comprises two classes.

Fig. 407. Diagram of The Structure of a phylactolaemate zooid (longitudinal section) (after Dogiel):

1 - intertentacular membrane; 2 - nephridium; 3 - ganglion; 4 - longitudinal lophophoral muscles; 5 - anus; 6 - pyloric region of The Stomach; 7 - circular muscles; 8 - caecum of the stomach; 9 - stomach retractor; 10 - funiculus;

11 - developing statoblast; 12 - young bud; 13 - septum; 14 - peritoneal epithelium; 15 - colony wall muscles;

16 - cardiac region of the stomach; 17 - trunk region of the coelom; 18 - Esophagus; 19 - Pharynx; 20 - coelomic Diaphragm; 21 - lophophoral region of the coelom; 22 - mouth; 23 - epistome; 24 - tentacles