INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE MULTICELLULAR ANIMALS (EUMETAZOA)

SECTION DIPLOBLASTICA, OR RADIATA

PHYLUM CNIDARIA, OR COELENTERATA

Aquatic, predominantly marine animals that are either sessile (attached to a substrate) or pelagic (free-swimming in the Water Column). They include both solitary and colonial forms, ranging in size from a few millimeters to several meters. Over 10,000 species are known (with 36 species represented in the fauna of Ukraine).

Cnidarians are characterized by radial Symmetry—typically tetramerous, hexamerous, or octomerous—diploblastic body Organization, and the presence of a Nervous system. They lack a true body cavity; their only internal cavity is the gastrovascular cavity, which is lined with gastrodermis and opens externally via the Mouth (hence the alternative name for the phylum—Coelenterata).

A defining feature of the phylum is the presence of stinging Cells, or cnidocytes (from the Greek knide meaning nettle) (Fig. 71). These pear-shaped cells contain a stinging capsule (nematocyst) enclosing a tightly coiled stinging thread. On the outer surface of The Cell, There is a slender sensory Hair called a cnidocil, whose ultrastructure resembles a long, immotile flagellum surrounded by fine cytoplasmic outgrowths—microvilli. Upon the slightest Touch to the cnidocil, the stinging thread rapidly everts, penetrates the body of prey or a predator, and injects the poisonous content of the capsule. The Base of the thread is barbed with posteriorly directed spines, which anchor it securely in the victim's Tissues. Stinging cells of this type are known as penetrants. Other types include volvents, which feature short, spine-free threads that entangle prey, and glutinants, which possess long, sticky threads used for adhesion. The primary Functions of cnidocytes are defense and prey capture. They are particularly abundant on the tentacles, where they form dense clusters called "batteries." The functioning of cnidocytes is autonomous and independent of The Nervous System. Certain animals (such as nudibranch Mollusks and turbellarians) that prey on Coelenterates do not digest these stinging cells; instead, the cells migrate to the animal's body surface and are repurposed for defense against attackers.

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Fig. 71. Types of cnidocytes in Hydra (adapted from Hadorn, with modifications):

a – penetrants at various stages of thread discharge; b – glutinants; c – volvents; d – cnidocyte batteries on the tentacles: 1 – operculum of the stinging capsule; 2 – cnidocil; 3 – stinging thread; 4 – Nucleus; 5 – expanded base of the stinging thread with bristles externally covered by stylets; 6 — stylets

Many cnidarian species possess a Skeleton that serves protective and often supportive functions.

A characteristic feature of many species in this group is the presence of two distinct life-cycle stages—the polyp and the medusa. Polyps are typically sessile, reproduce predominantly asexually, and often form colonies. Medusae are free-swimming, solitary animals that reproduce primarily sexually. The life cycle often involves a regular alternation of polyp and medusa generations, known as metagenesis, though partial reduction of one generation—gonogenesis—can also occur. Frequently, one of the generations may be completely absent (Fig. 72). A notable characteristic of cnidarian polyps is their high capacity for regeneration.

The phylum Cnidaria is divided into two subphyla.

Fig. 72. Generalized diagram of coelenterate life cycles:

1 – polyp; 2 – medusa; 3 – Gametes; 4 – zygote; 5-6 – Cleavage; 7 – morula;

8 – blastula; 9 – delamination; 10 – immigration; 11 – invagination; 12 – parenchymula;

13 – planula (R! – reduction division)