INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM METAZOA

SUBKINGDOM EUMETAZOA

BRANCH DIPLOBLASTICA, or RADIATA

PHYLUM CNIDARIA, or COELENTERATA

SUBPHYLUM ANTHOZOA

Marine colonial, less commonly solitary organisms whose life cycle lacks metagenesis (featuring only the polypoid generation). Approximately 5,000 species of these animals are known; four species occur in the Black Sea and one in the Sea of Azov.

Most species possess a Skeleton (horny or calcareous). An ectodermal Pharynx is present, and the gastral cavity is partitioned into chambers. Gonads develop from the endoderm. Discrete Muscle Cells are present. Development proceeds with metamorphosis.

Structure. The body of an individual polyp is cylindrical (Fig. 91). Solitary polyps attach to the substrate by means of a pedal disc, whereas in colonial forms, the lower portion of the individuals is embedded within the common body of the colony (the coenenchyme). At the opposite end lies the oral disc, bearing a slit-like Mouth surrounded by tentacles. Externally, the body is covered by an Epithelial Tissue layer, beneath which lie layers of longitudinal and circular muscle cells. Muscle cells are also present beneath the gastrodermis. The mesoglea is well developed. The Skeletal structure may vary.

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Fig. 91. Diagram of The structure of Alcyonium (after Marfenin):

a - individual polyp; b - section of the colony; c, d - transverse sections of a colony section at the level of the pharynx and Stomach, respectively (after Dogel):

1 - pharynx; 2 - gastral cavity; 3 - canals connecting the polyp cavities; 4 - coenenchyme; 5 - gonads;

6 - mesenterial filaments; 7 - polyp; 8 - chamber; 9 - gastrodermis; 10 - muscular ridge;

11 - stomach; 12 - siphonoglyph; 13 - mesoglea with spicules

The ectodermal pharynx projects into the gastral cavity, which is lined with gastrodermis. The pharynx is slit-shaped and compressed. Grooves known as siphonoglyphs are located on one or both sides of the pharynx; their cells bear very long cilia. These cilia beat continuously to drive Water into the gastral cavity; when There are two siphonoglyphs, one directs the water current inward and the other outward.

The gastral cavity is partitioned into chambers by septa (partitions) that extend from the inner surface of the body wall. These are folds of the gastrodermis containing a layer of mesoglea between its two cellular layers, complete with muscle cells that form a longitudinal muscular ridge. In the upper part, the septa typically fuse with the pharyngeal walls; in the lower part, they hang freely into the gastral cavity, leaving its central region (the "stomach") undivided (Figs. 91, 92). Most of The surface of the septa, like the inner surface of the body wall, consists of ciliated epithelial cells that perform hydrokinetic Functions and are capable of phagocytosis. The free edges of the septa projecting into the gastral cavity feature corrugated thickenings—mesenterial filaments—composed of glandular and phagocytic cells. In many species, the lower part of the polyp sees the mesenterial filaments extend into long, thin internal tentacles (acontia) that also bear cnidocytes ( stinging cells). In some species, acontia can be everted through the mouth or specialized pores and serve for defense and Digestion. Swallowed prey enters the central region of the gastral cavity (the "stomach"). The mesenterial filaments close around the prey, acontia entangle it, and a temporarily enclosed cavity is formed wherein both extracellular and intracellular digestion take place. Subsequently, the septa separate, and the nutrients resulting from digestion are distributed to the periphery of the body through the action of the epithelial cilia.

Coral polyps include active filter feeders, highly specialized predators, and species that combine both feeding strategies. They are also capable of absorbing organic matter directly from the water, while most of them obtain nutrients from symbionts—zooxanthellae (the dinoflagellate Symbiodinium microadriaticum) living within the gastrodermal cells.

The Nervous system of coral polyps has the form of a diffuse plexus; differentiated Sense Organs are absent, their function being performed by isolated sensory cells.

Reproduction. Coral polyps reproduce both asexually and sexually. Asexual reproduction occurs via budding, transverse or longitudinal fission of individuals, and laceration (reproduction through the detachment of a piece or group of cells from the polyp—most commonly its basal part—from which a new individual develops). In addition, colonial scleractinians are capable of colony fragmentation and the healing of its fragments, or The Emergence of individual naked polyps from the exoskeleton into the water Column followed by settlement on a substrate, after which a new colony begins to develop, and so on.

Fig. 92. Sea anemone:

a - structural diagram (after Naumov et al.); b - transverse section through a septum (after V. Beklemishev):

1 - Chambers of the gastral cavity; 2 - acontium; 3 - pores for the exit of acontia; 4 - radial Muscles; 5 - siphonoglyph; 6 - circular muscles;

7 - Pores in the walls of the septa; 8 - pharynx; 9 - mesenterial filaments; 10 - longitudinal muscles; 11 - septa with muscular ridges;

12 - mesenteric filaments with digestive and stinging cells; 13 - septum wall with transport gastrodermal epithelium;

14 — mesoglea

During sexual reproduction, gonads develop in the gastrodermis of the septa. Among coral polyps, there are both dioecious forms and hermaphrodites. In most coral polyps, sex cells are released into the water through the mouth, and Fertilization is external. A planula larva develops from the fertilized egg. It swims, feeds on small planktonic organisms, and then settles on a substrate to transform into a polyp. In some species, a tentacled larva—an actinula—hatches from the egg at later Selection/3.html">Stages of development and resembles a polyp. There are also species of coral polyps in which egg fertilization occurs inside the maternal Organism, where the Initial Stages of larval development also take place.

The subphylum Anthozoa is divided into four classes.