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

PHYLUM BRYOZOANS (BRYOZOA)

This phylum includes a large group of microscopic (from 0.3 to 5 mm) aquatic benthic animals that lead a colonial lifestyle. In total, about 4,000 extant and 15,000 extinct species are known. Most bryozoans inhabit the World Ocean, with far fewer species living in fresh Water.

The shape of the colonies is extremely diverse; most of them are tree- or shrub-like, but There are also leaf-like forms, as well as those that creep along the substrate or encrust various objects. Colony sizes are usually small—up to a few centimeters—but much larger ones, up to a meter or more, also occur.

The foundation of the colonies consists of the so-called feeding individuals, or zooids. In addition to these, marine bryozoans have several types of modified zooids that perform other Functions.

The body walls of individual zooids consist of a chitinous cuticle, an epidermal layer, more or less developed layers of circular and longitudinal Muscles, and the peritoneal epithelium. In many marine forms, the cuticle is impregnated with calcium carbonate.

The walls of the anterior part of the body, or polypide, are covered with a very thin, elastic cuticle, which allows the polypide to be easily retracted into the posterior part of the body, or cystid. At the anterior end of the polypide is the tentacular apparatus, which has a different Structure in different species.

The body cavity (coelom) is divided by a septum of peritoneal epithelium into two compartments—a small ring canal that branches into the tentacular apparatus, and a large zooid cavity, which in freshwater bryozoans communicates with the common coelomic cavity of the entire colony. The coelomic fluid contains Proteins and free Cells—coelomocytes.

In addition to the body wall muscles, The Muscular System of bryozoans includes a significant number of specialized Muscle bundles, the largest of which are the muscles that ensure the rapid retraction of the polypide into the cystid.

The Digestive System is loop-shaped; the Mouth is located inside the tentacular crown and leads into the Pharynx; this is followed by the Esophagus, which connects to an endodermal Stomach of rather complex structure; from The Stomach, a thin endodermal midgut extends upward to the anterior end of the body, ending in a rectum with an anus that opens outside the tentacles.

Specialized excretory Organs in the form of two coelomoducts are present only in freshwater bryozoans. The majority of excretory products in bryozoans are removed by amoebocytes floating in the coelomic fluid. Filled with waste products, the amoebocytes pass outward through the epidermis of the tentacles and the intestinal walls.

There is no Circulatory system; Respiration occurs through the body surface, primarily through the thin integument of the tentacles.

The Nervous system consists of a supraesophageal ganglion located between the mouth and the anus, from which nerves extend to the tentacles, body walls, and Internal Organs. Coordination of all zooids in the colony is enabled by the presence of a colony-wide nervous plexus.

The vast majority of bryozoans are hermaphroditic. The structure of the Reproductive System is very simple. Germ Cells develop from specialized cells of mesodermal origin located beneath the peritoneal epithelium. Fertilization is internal; motile spermatozoa actively move from one colony to another and freely penetrate the coelom of zooids with mature eggs.

Cleavage of the eggs is complete and in many cases equal, but it is not highly ordered. The eggs hatch into a larva, which has a different structure in representatives of different classes.

In addition to sexual reproduction, bryozoans also reproduce asexually, particularly by budding, which is how colony growth occurs.

The phylum Bryozoa includes two classes: Phylactolaemates (Phylactolaemata) and Gymnolaemates (Gymnolaemata).