Coelenterata

COELENTERATA, a phylum of coelenterate animals; lower invertebrates belong to it. The body of Coelenterata consists of a two-layered sac; its wall is formed by the ectoderm and entoderm, between which lies a basal membrane or more or less powerful accumulations of mesoglea—a gelatinous substance characteristic of jellyfish. The sac-like body of Coelenterata has a single opening leading into the digestive or gastrovascular cavity. Around this opening lie finger-like protrusions—tentacles. Due to their arrangement, the body of Coelenterata is constructed according to radial symmetry. The ectoderm of Coelenterata consists of various cells: indifferent, glandular, epithelial-muscular, sensory, stinging (nematocysts), and others. Stinging cells are highly characteristic of Coelenterata, and based on this feature, the subphylum Cnidaria is established. Stinging cells (see figure) are poisonous apparatuses due to the presence within them of a special capsule with an inverted thread inside, which, upon irritation,

Stinging cells of Hydra; on the left—charged, on the right—having ejected the thread: 1—nucleus; 2—capsule; 3—sensory rod; 4—spiral thread; 5—supporting rods. (According to Schulze.)

is ejected outward and pierces the animal that has touched it. The entoderm of Coelenterata consists of glandular and epithelial-muscular cells. Examples of Coelenterata are the hydra, various colonial hydroids, siphonophores, scyphozoan jellyfish, coral polyps, and ctenophores (subphylum Ctenaria). For many Coelenterata, reproduction with an alternation of sexual and asexual generations (metagenesis) is characteristic. A very common form of reproduction for Coelenterata is budding, leading to the formation of often huge colonies (e.g., coral reefs). The vast majority of Coelenterata inhabit seawater; few species live in fresh water. In medical terms, the toxicity of Coelenterata is of significance to humans. Stinging cells cause skin irritation through their action: redness, itching, pain, and a burning sensation (jellyfish and especially some siphonophores). Upon repeated exposure, phenomena of allergic intoxication may develop. Coelenterata can cause occupational diseases; such is the "disease of sponge divers." Sea anemones often sit on sponges, and divers who tear off sponges with their bare hands are subjected to the repeated action of the stinging cells of the sea anemones, which leads to ulceration of the skin of the hands. There have been attempts to use jellyfish for the treatment of rheumatism (mixing jellyfish into therapeutic mud). The active principles of sea anemones are considered to be

thalassin and congestin.