INVERTEBRATE ZOOLOGY IN THREE VOLUMES - BOOK 2 - H.Y. Shcherbak - 1996

PHYLUM ANNELIDA

CLASS POLYCHAETA

SUBCLASS SEDENTARIA

Representatives of this subclass inhabit marine sediments and various underwater objects. As a rule, through the secretions of their dermal glands, they form temporary or permanent protective tubes made of organic matter, often impregnated with calcium carbonate.

The HEAD lobe is usually poorly developed or completely reduced; its appendages are transformed into cephalic tentacles or gills that protrude from the tube but can be retracted into it rapidly. The body is mostly heteronomous: it consists of two or three tagmata (regions), the segments of which differ significantly from one another; nephridia are present only in some of the segments. Both burrowing and sedentary forms are known.

The subclass Sedentaria includes four orders: Drilomorpha, Spiomorpha, Terebellomorpha, and Serpulimorpha. Among them, two ecological groups are distinguished: burrowing forms that feed mainly on organic matter in the sediment, and tube-dwelling forms that feed on small particles captured from the Water by their tentacular apparatus.

The first group comprises Representatives of the order Drilomorpha, such as lugworms; the second group is formed predominantly by members of the other three orders. Among the burrowing forms, lugworms are the best known—large worms (up to 30 cm in length). For instance, Аrеnісоlа marina (Fig. 15, a) lives in the sandy littoral zone (the coastal part of the seabed exposed during low tide) in the White and Barents Seas, while A. branchialis inhabits sandy shallows in the Black Sea. The body of the lugworm is divided into three sharply distinct regions: the anterior region (six segments) with small parapodia but no gills, the middle region (13 segments) with parapodia bearing gills, and the posterior region, comprising a variable number of segments (depending on the individual) without any appendages. This is a striking example of heteronomous segmentation. The coelom in lugworms has transverse partitions (septa) only in the anterior part; elsewhere, the coelom is continuous. This accounts for the peristaltic Movements of the worm's body: coelomic fluid is constantly pumped from back to front by means of Muscles, accompanied by the successive expansion and constriction of segments. In this way, the worm burrows into the sediment and widens the walls of its burrow. The worm swallows sand containing detritus and small organisms, which it feeds on by everting and retracting the buccal region. The sand with indigestible residues is egested at the opposite end of the burrow. The lugworm constructs a U-shaped tube in the sand, the walls of which are cemented with dermal gland secretions. A pile of sand ejected by the lugworm is always heaped near the posterior end of the established burrow, whereas a funnel forms at the anterior end, where sand with various organic inclusions is drawn in. The peristaltic movements of the worm create a continuous flow of water through the burrow, facilitating gill ventilation. As a rule, lugworms live in colonies occupying large areas.

Most species of Sedentaria build tubes that attach to rocky bottoms, cliffs, mollusk shells, crab carapaces, tubes of other polychaetes, and Algae. In many species, foreign particles—such as sand grains and fragments of mollusk shells—are glued to the organic horn-like substance secreted by the dermal glands, which increases the durability of the tube; in some forms, the organic foundation of the tube is impregnated with calcium carbonate.

Tube-dwelling polychaetes are characterized by the presence of numerous appendages at the anterior end of the body, which protrudes from the tube: these serve for feeding, Respiration, and defense. Most commonly, they are modified palps, sometimes antennae of the first segment. The part of the body located inside the tube is divided into two regions: the anterior region, which is more muscular, covered with glandular shields involved in tube construction, and possesses more or less developed parapodia; and the posterior region, with reduced parapodia, which primarily performs reproductive Functions.

Specialization of the cephalic appendages and the division of body segments into regions vary across different orders and families. In representatives of the order Terebellomorpha, the prostomium bears numerous (sometimes over 100) long, thread-like tentacles designed for gathering food (Fig. 15, c). Protruding from the tube, they spread out in all directions over The surface of the sediment. The ventral side of the tentacle features a longitudinal ciliated groove along which food particles are conveyed toward the Mouth. The second group of appendages consists of gills that are thread-like but shorter than the tentacles, located on the parapodia of the anterior segments. Serpulimorpha also possess well-developed cephalic appendages—"gills"—of a different Structure. These consist of a pair of lobes from the anterior margin of which extend numerous pinnate tentacles covered with ciliated epithelium (Fig. 15, g, d). The head gills, through their movement, direct water along with small organisms toward the worm's mouth.

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Fig. 15. Sedentary polychaetes:

a - general view of the lugworm АІеnісоlа marina; b - segments of the middle part of its body; c - inside an organic tube encrusted with small pebbles; g, d - Serpula and Sptrorbis in calcareous tubes, respectively; 1 - caudal region; 2 - gills; 3 - notopodium; 4 - anterior segments; 5 - peristomium; 6 - parastomium; 7 - buccal region; 8 - Pharynx; 9 - neuropodia; 10 - anus; 11 - tentacles; 12 - operculum

The tubes of sedentary polychaetes often blanket the entire seabed at shallow depths. Frequently, they fuse together to form massive reefs. For instance, the tubes of Sabellaria alveolata form a continuous blanket over coastal rocks off the shores of Southern England, creating settlements resembling coral reefs. Other species, such as those from the family Serpulidae, form robust aggregates of calcareous tubes comprising several hundred individuals. These worms are found among coral reefs or attached to mollusk shells. Many serpulid species foul ship hulls and underwater structures by attaching to them. They are among the most beautiful polychaetes, with their tentacles colored in very bright, vibrant hues.

In the sea, spiral calcareous tubes housing small (up to 10 mm) species of the genus Spirorbis are frequently found on brown and red algae (Fig. 15, d). Their palps have transformed into branched head gills designed for both respiration and water filtration as well as the capture of food particles; a specialized operculum is also formed from the palps.

When in danger, the worm retracts its gills into the tube and closes the opening with the operculum.

Representatives of the family Sabellidae live in tubes on the bottom; their head gills are highly branched and also brightly colored. They feed not only on living organisms but also on the remains of small planktonic animals that continually settle to the bottom. Species of the genus Manayunkia inhabit the estuaries of rivers flowing into the Azov and Caspian Seas. The polychaetes Hypania invalida and Hyponicola kowalewskn form massive settlements in the benthos of the Dnieper, Danube, and Southern Bug rivers. The population density of Н. kowalewskii in the Dnieper estuary reaches 2.5 thousand individuals per 1 m².

Fossilized tubes of sedentary polychaetes, as well as their jaws and setae, are well preserved in the fossil record; well-preserved body imprints of these animals are also known. Reliable finds of Sedentaria tubes date back to Cambrian deposits. During the Cretaceous period, serpulites formed—limestone rocks consisting entirely of sedentary polychaete tubes and forming layers up to 50 cm thick.