Quiz 13: Radiate Animals
The utility of radial symmetry for sessile and free-floating animals is as follows: • The presence of radial symmetry helps the animal in reaching out in all the directions from one point. Example: During feeding, the animal can make use of being radially symmetrical as these animals lack a high degree of controllable movements. • The presence of radial symmetry helps the animal to receive stimuli from all the directions; thereby, building up a strong and efficient defence mechanism. Hence, both sessile and free-floating animals have advantage of having radial symmetry.
The characteristics of phylum Cnidaria that distinguish it from other phyla are as follows: • Cnidocytes are present which contain stinging cells called nematocysts. • Two types of individuals: polyp and medusa. • Extensible tentacles usually encircle mouth or oral cavity. • Asexual reproduction by budding in polyps forms clones; some forms exhibit polymorphism. • Sense organs include well developed statocysts (organs of balance) and ocelli (photosensitive organs); complex eyes in members of Cubozoa.
Taxonomic classes of phylum Cnidaria: Class Hydrozoa - • They can be either solitary or colonial; • Asexual polyps and sexual medusae, although any one type may be suppressed; • Hydranths with no mesenteries; • Medusae with a velum; • Present in both fresh water and marine. • E.g. - Hydra, Obelia, Physalia, Tubularia. Class Scyphozoa - • They are solitary • The polyp stage reduced or absent; • The bell-shaped medusae is present without velum; • The gelatinous mesoglea is much enlarged; • The margin of the bell or umbrella typically has eight notches that are provided with sense organs; • All are marine • E.g. - Aurelia, Rhizostoma , Cassiopeia. Class Staurozoa - • They are solitary; • Only polyps are present; medusae are absent; • The surface of the polyp is extended into eight clusters of tentacles surrounding mouth; • attachment is via adhesive disc; • All are marine • E.g. - Haliclystis, Lucernaria. Class Cubozoa - • They are solitary; • The polyp stage is reduced; the bell-shaped medusae is square in cross section, • A tentacle or a group of tentacles hanging from a bladelike pedalium at each corner of the umbrella; • margin of umbrella entire, without velum but with velarium; • All are marine • E.g. - Chironex, Tripedalia, Carybdea, Chiropsalmus. Class Anthozoa - • All polyps are present and medusae are absent; • They can be either solitary or colonial; • The gastrovascular cavity is subdivided by at least eight mesenteries or septa bearing nematocysts; • The gonads are endodermal; • all are marine • E.g. - Tubipora, Cerianthus, Metridium.