COMPREHENSIVE LECTURE NOTES IN ZOOLOGY

Module 2. SUWKIND MULTICELLULAR ANIMALS (METAZOA). SECTION PRIMITIVE METAZOA (PROMETAZOA). LOWER EUMETAZOA (EUMETAZOA)

Content Module 2.2. Lower Eumetazoa

Topic 2.2.2. Phylum Ctenophora (Comb Jellies)

General characteristics of the phylum. Origin, evolution, paleontological records, contemporary diversity, geographic distribution, and their ecological and economic significance. Overview of major taxonomic groups crucial for natural ecosystems and human life. Class Ctenophora. Exclusively marine animals. Approximately 120 species, the majority inhabiting tropical seas; in polar regions, certain species form massive aggregations. Only a single species, Pleurobrachia rhodopis, is found in the Black and Azov Seas. Predominantly free-swimming predators, though crawling and sessile detritivorous species are also known. Sizes range from 2–3 mm (Tinerfe cyanea) to up to 2.5 m (Venus' girdle

- Cestus veneris). Radially symmetrical, diploblastic animals (with a rudimentary mesoderm), lacking stinging Cells (nematocysts), polypoid developmental stages, and metagenesis. The body plan of ctenophores is characterized by a combination of eight-ray and biradial Symmetry. Locomotion is achieved via modified cilia arranged as comb plates (ctenes). Hermaphroditic with external Fertilization and direct development. The phylum comprises a single class, Ctenophora.

Class Ctenophora. The body of ctenophores is sac-like, oval, spherical, or pear-shaped. The oral pole features the Mouth opening, while the opposite, aboral pole bears the aboral organ. The main body axis passes through both poles. Extending meridionally across the body surface (from the oral to the aboral pole) are eight longitudinal ridges, or Ribs, bearing transverse comb plates formed by fused cilia. These are the longest cilia in the animal kingdom (measuring several millimeters in length) and contain three central microtubules. When beating, the comb plates refract light to create an iridescent effect. Serving as the primary locomotory Organs, they propel the animal forward with the oral pole leading. Most ctenophores possess two tentacles, sometimes significantly longer than the body, which can be retracted into specialized tentacular sheaths. One surface of each tentacle bears branches covered with specialized adhesive cells (colloblasts) unique to ctenophores.