SUPPORTING LECTURE NOTES IN ZOOLOGY

Module 4. MOLLUSCS, CHAETOGNATHS, PHORONIDS, BRYOZOANS, BRACHIOPODS, POGONOPHORANS, HEMICHORDATES, ECHINODERMS

Content Module 4.1. Molluscs, Chaetognaths, Phoronids, Bryozoans, Brachiopods, Pogonophorans

Topic 4.1.2. Phylum Chaetognatha (Arrow Worms)

General characteristics of the phylum. Origin, evolution, paleontological Background, modern diversity, geographic distribution, and their role in nature and human life. Overview of major taxonomic groups essential to human life and the functioning of natural ecosystems. Class Chaetognatha.

Marine animals, mostly leading a pelagic lifestyle. They possess an elongated, bilaterally symmetrical, often transparent body divided into HEAD, trunk, and tail regions. The Mouth is located at the anterior end of the ventral side of the head, surrounded by a powerful grasping apparatus. On the sides and at the posterior end of the body

- fins. The body is covered with a thin cuticle and a single-layered epithelium, featuring multi-layered areas in some regions. Adult animals have a primary body cavity (pseudocoelom), although a true coelom forms during embryonic development. The digestive tract is a straight tube, with the anus situated at the boundary between the trunk and tail regions. Specialized excretory, respiratory, and circulatory systems are absent. The Nervous system consists of a Brain (supraesophageal ganglion), several smaller ganglia in various PARTS OF THE body, a prominent ventral ganglion, and long connectives linking them. Hermaphrodites; development is direct, without metamorphosis. The phylum Chaetognatha comprises a single class of the same name.

Class Chaetognatha, or Arrow Worms.

Exclusively marine predators that tolerate desalination poorly, though a few species have adapted to reduced Water salinity. Around 150 species, of which three inhabit the Black Sea (including the endemic Sagitta euxina) and one is found in the Sea of Azov. The bulk of species inhabit the water Column at various depths, remaining in constant motion. Some species exhibit adaptations for floating, such as broad fin-like appendages (genus Krohnitta), or weight reduction through the regression of denticles, bristles, and Muscle tissue, which—along with an increased body volume—decreases specific gravity (genus Hassagitta), among others. Nine species of the family Spadellidae are benthic, inhabiting shallow waters where they maneuver among Algae and rocks, as well as greater depths (down to 1000 m). Sizes range from 0.5 to 9 cm. The smallest species are benthic, while the largest are northern nektonic forms. As a rule, they are colorless (making them difficult to spot in the water). Reddish, orange-brown, and yellow-green arrow worms occur among benthic species. The body is covered with a thin cuticle, beneath which lies a single-layered (plaque-like) epithelium; Cell margins are wavy and interlock such that the protrusion of one cell fits into the depression of the neighboring one. This serrated interlocking provides the epithelium with elasticity and flexibility, preventing the integument from tearing during rapid movements.