LECTURE NOTES AND STUDY GUIDE IN ZOOLOGY

Module 2. SUBKINGDOM MULTICELLULAR ANIMALS (METAZOA). DIVISION PROMETAZOA. LOWER METAZOA (EUMETAZOA)

Content Module 2.4. Scalidophorans and Cephalorhynchs

Topic 2.4.2. Phylum Cephalorhyncha

General characteristics of the phylum. Origin, evolution, paleontological data, modern diversity, geographical distribution, and their role in nature and human life. Overview of major taxonomic groups important for human activities and the functioning of natural ecosystems. Class Priapulida. Class Kinorhyncha. Class Nematomorpha. Class Loricifera.

Worm-like organisms; most species are marine, while only Representatives of the class Nematomorpha (Gordiacea) spend part of their life cycle in marine and freshwater habitats, and another part inside Arthropods. Sizes range from 25 µm to 1.5 m. Cephalorhynchs share a similar level of Organization with rotifers, acanthocephalans, gastrotrichs, and nematodes. The body is divided into two regions: the HEAD (proboscis) and the trunk. The proboscis comprises an oral cone, a middle section with backward-pointing hooks (scalids), and a neck region. Ciliature is absent; the body is covered by a cuticle and features a cutaneous-muscular sac enclosing a primary body cavity (schizocoel). The digestive tract is complete. The excretory system is of the protonephridial type, though completely absent in some representatives. The Nervous System consists of a circumoesophageal nerve ring and an unpaired ventral nerve cord. These animals are dioecious, with radial and nearly equal Cleavage. The blastopore develops into the anus, while the Mouth opens at the anterior end of the embryo independently of the blastopore. Development proceeds through metamorphosis, hatching into a larva that differs significantly from the adult. The phylum Cephalorhyncha includes four classes: Priapulida, Kinorhyncha, Nematomorpha (Gordiacea), and Loricifera.

Class Priapulida. Marine benthic (bottom-dwelling) animals inhabiting temperate zones and abyssal depths of the World Ocean. 15 species are known.

Class Kinorhyncha. Marine benthic animals inhabiting marine mud, sand, and Algae. Found in all seas and oceans, with the majority of species inhabiting sublittoral and littoral zones. Comprising about 100 species, with 14 species found in the Black Sea, three of which are endemic (Centroderes spinosus, Semnoderes ponticus). The body is elongate, ranging from 0.18 to 1 mm in length, and consists of a short proboscis and an elongated trunk. The proboscis region features an oral cone bearing forward-directed cuticular outgrowths, a middle section armed with backward-curved spine-like scalids, and a smooth neck region. The trunk is covered by a cuticular lorica composed of individual segments called zonites, most commonly numbering 11.

Class Nematomorpha (Gordiacea). Parasitic worms characterized by larval parasitism. Over 300 species. The fauna of Ukraine includes 2 species: the freshwater Gordius aquaticus and Nectonema agile (found in Black Sea crustaceans). Free-living habits are restricted to newly hatched larvae and adult worms. They inhabit marine and freshwater environments. Larvae parasitize insects, whereas marine species parasitize crustaceans. The body is long and thread-like, measuring 40–50 cm in length, and up to 1.5 m in some species (with a width of 1–3 mm). Because of this, the class is commonly known as "hairworms" or "horsehair worms." Adults lack a proboscis, which is present exclusively in the larval stage. The anterior end of the body is rounded, while the posterior end in males is bifurcated.

Class Loricifera. Marine benthic animals inhabiting the interstitial spaces between sand grains. Distributed in seas and oceans across various depths (from 20 to 500 m). Two families, three genera, and nine species have been described, the first of which is Nanaloricus mysticus. Microscopic organisms with a body length of 225–385 µm and a width of 90 µm. The body is divided into a proboscis and a trunk region. The proboscis region comprises an oral cone surrounded by eight to nine pointed stylet-like spines, a middle section armed with nine rows of scalids, and a neck region with neck scalids. The trunk is enclosed in a cuticular armor (lorica) consisting of six plates: one dorsal, one ventral, and four lateral plates. The plates of the armor feature a delicate honeycomb-like sculpture. Gland ducts open through pores On the surface of the armor. The posterior part of the lorica bears specialized glands that also function as Sensory Organs, known as flosculi.