INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE MULTICELLULAR ANIMALS (EUMETAZOA)

DIVISION TRIPLOBLASTIC (TRIPLOBLASTICA) OR BILATERAL (BILATERIA) ANIMALS

SUBDIVISION SPIRALIA (SPIRALIA)

PHYLUM GASTROTRICHA (GASTROTRICHA)

The phylum comprises about 400 species of small animals (0.5–1.5 mm, rarely up to 4 mm in length) inhabiting marine and freshwater environments. In Ukraine, approximately 40 species have been found in freshwaters and 20 in the Black Sea.

Gastrotrichs are characterized by a combination of ciliary covering and cuticle, a pseudocoel (schizocoel) body cavity, a complete digestive tract, a Pharynx with a triangular lumen and pores connecting it to the external environment, a protonephridial excretory system, and Hermaphroditism.

The phylum includes two classes: Macrodasyoidea and Chaetonotoidea.

Anatomy and Morphology. The body of gastrotrichs is flattened on the ventral side and convex on the dorsal side, terminating in a more or less elongated fork-like caudal appendage. Species of the Class Macrodasyoidea, which inhabit narrow capillary spaces between sand grains on marine beaches, have a narrow, elongated body shape, whereas species of the class Chaetonotoidea—inhabitants of freshwaters living On the surface of substrate and aquatic vegetation—have a shortened, flask-shaped body (Fig. 162).

Fig. 162. Gastrotrichs (after Malakhov): a - Chaetonotus (class Chaetonotoidea); b - Urodasys (class Macrodasyoidea)

Integument. The ventral side bears numerous cilia, which gives the class its name (from Greek gaster – Stomach, belly, and thrix – Hair). The body is covered by a thin, elastic cuticle, which in chaetonotids thickens on the dorsal side and bears numerous scales and spines. The cuticle also covers the bases of the cilia without hindering their beat. Characteristic Features of gastrotrichs are the so-called adhesive tubes (Fig. 163). These are specialized glandular-Sensory Organs in the form of short tubular outgrowths of the outer cuticle, open at the tips. They consist of three Cells: a supporting Cell, a glandular cell, and a sensory cell with a flagellum.

The glandular cell secretes an adhesive substance that allows the animal to attach to the substrate or crawl along it by alternately fixing its anterior and posterior ends. The sensory cell possesses a long, non-motile flagellum—a sensory process of a nerve cell—while its second process connects to the nerve cord; thus, the adhesive tubes also function as mechanoreceptors (tactile organs). Beneath the cuticle lies the hypodermis, which has the appearance of an embedded epithelium.

Musculature. Gastrotrichs lack a continuous Skin-Muscle sac: the Muscles, which are obliquely striated in Structure, form separate bands. Longitudinal muscles are the most developed, extending in two wide bands along the sides of the body. Macrodasyoids also possess circular muscles. In addition, all gastrotrichs have dorsoventral muscles as well as gut musculature.

Fig. 163. Diagram of gastrotrich anatomy (after Malakhov)

a, b - longitudinal and transverse sections, respectively: 1 - Mouth; 2 - sensory bristles; 3 - sensory pits; 4 - nerve ring; 5 - pharynx;

6 - ventrolateral nerves; 7 - adhesive tubes; 8 - pharyngeal pores; 9 - Testes; 10 - protonephridia; 11 - midgut;

12 - Ovaries; 13 - copulatory organ; 14 - hindgut; 15 - anus; 16 - cuticle; 17 - epithelium; 18 - circular musculature;

19 - Y-organ cells; 20 - longitudinal muscles; 21 - ventral ciliary band

Body Cavity. The schizocoel is poorly developed: Internal Organs are densely packed against one another, with only narrow slits between them. In macrodasyoids, a so-called Y-organ is located on either side of the gut, maintaining high body turgor. It consists of two rows of large, heavily vacuolated mesodermal cells that join together at the posterior end of the body.

The Digestive System consists of a relatively long pharynx with a triangular lumen, a midgut, and a short hindgut terminating in the anus.

Representatives of the class Macrodasyoidea possess pharyngeal pores—structures unique among invertebrates—that connect the pharyngeal cavity to the external environment. Gastrotrichs feed on small organisms such as Bacteria, Protozoa, and unicellular Algae. Food is swept toward the mouth by the beating of cilia.

The excretory system is represented by paired protonephridia, which also perform an osmoregulatory function, a role of particular importance for species inhabiting the marine littoral zone with its sharp fluctuations in Water salinity.

Respiration in gastrotrichs occurs through the body surface.

The Nervous System consists of a circumpharyngeal nerve ring giving rise to two longitudinal cords that extend along the sides of the ventral region of the body. The nerve ring is composed mainly of nerve fibers, while the Nerve Cells form loose masses located anteriorly and posteriorly to the ring. True ganglia are absent in gastrotrichs.

Sense Organs are represented by two pairs of ciliated pits at the anterior end of the body acting as chemoreceptors, adhesive tubes mentioned earlier (mechanoreceptors), and individual sensilla on the anterior end of the body. Some species possess simple eyes situated above the circumpharyngeal nerve ring.

Reproductive System. Marine gastrotrichs (order Macrodasyida) are hermaphroditic. Their Gonads are paired, and the gonopore is single. Fertilization is internal.

Reproduction. Freshwater species of the order Chaetonotida are predominantly represented by females that reproduce parthenogenetically. Features of embryonic development include bilateral, determinate Cleavage of the egg. Postembryonic development is direct, with a fully formed juvenile hatching from the egg.