INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM METAZOA

SUBKINGDOM EUMETAZOA

DIVISION TRIPLOBLASTICA, OR BILATERIA

SUBDIVISION SPIRALIA

PHYLUM PLATHELMINTHES

SUBPHYLUM TURBELLARIA, OR ARCHIDERMATA

SUPERCLASS NEOPHORA

CLASS SERIATA

This is the most species-rich Class. They are predominantly freshwater animals, and more rarely marine (usually inhabiting the littoral zone); some tropical species live in damp soil. Their sizes range from a few millimeters to tens of centimeters. They often feature bright coloration. The middle section of the intestine is represented by three branched arms. They are predatory animals that hunt oligochaetes, small Mollusks, and Arthropods, locating their prey using chemoreceptors—auricular Organs and ciliated pits at the anterior end of the body.

In the freshwater bodies of Ukraine, under stones and fallen leaves, one can find the milk-white planarian (Dendrocoelum lacteum), milky-white in color (which gave the species its name), up to 25 mm in length. Another typical inhabitant of Ukrainian freshwaters is the black polycelis (Polycelis nigra), up to 12 mm long, with numerous ocelli along the sides of the anterior body region (Fig. 120).

Lake Baikal is home to over 30 endemic planarian species (endemics are organisms whose distribution is restricted to a specific, relatively small area). They are brightly colored, possess adhesive pits and suckers used to attach to the substrate, and have A large number (several dozen) of eyes. Among them are large species, such as Baikaloplana valida, reaching up to 40 cm in length (Fig. 120, g).

A significant number of planarian species inhabit the damp soil of tropical and subtropical forests. In terrestrial planarians, cilia may persist only on the ventral side of the body, which transforms into a specialized creeping sole. They move by the contraction of the Skin-muscular sac and feed on terrestrial invertebrates. Some species of land planarians are even capable of climbing trees. Terrestrial tropical planarians can be introduced to other countries along with plants and soil, subsequently spreading from greenhouses and botanical gardens into natural ecosystems. It is believed that Bipalium kewense (up to 25 cm long) was introduced to European countries with a warm climate from Indochina in precisely this manner.

Fig. 120. Seriate turbellarians (from Ivanov): a — Dendrocoelum lacteum; б — Polycetis nigra; в — Dugesia tigrina; г — Baikaloplana sp.

Some species are capable of prolonged starvation. For instance, Dugesia tigrina can survive up to one year without food, during which its size decreases hundreds of times and some of its organs degenerate. Upon the resumption of feeding, the organs regenerate, and the worms regain their reproductive capacity.

In addition to sexual reproduction, freshwater inhabitants are capable of Vegetative Reproduction via transverse fission. In Dugesia tigrina, a new individual can develop from 1/300th of the mother's body. This species, widespread throughout Europe, was introduced here from North America.