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

PHYLUM ANNELIDA

CLASS POLYCHAETA

SUBCLASS ERRANTIA

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This subclass comprises predominantly free-living predatory polychaetes, or more rarely detritivores. They possess a prostomium with well-developed Sense Organs, a mostly homonomous metameric trunk, and parapodia running along the entire length of the body; gills are also frequently well-developed. The majority of errant polychaetes live on the bottom surface, although some species construct mucus tubes or burrow into the sediment; swimming forms are also known.

This subclass includes the orders Phyllodocemorpha, Nereimorpha, and Eunicemorpha, which exhibit various life forms (Fig. 14, a, b). Pelagic species inhabiting the Water Column possess well-developed sense organs, particularly large eyes, and robust jaws within the buccal region. Their parapodia are modified into peculiar swimming fins. These are mostly carnivorous species, such as *Tomopteris pacifica*, which inhabits the northwestern Pacific Ocean. Its body is transparent, the parapodia lack setae, and one pair of antennae is exceptionally long. Another species, *Vanadis minuta*, features a pair of finger-like appendages on the anterior part of its buccal region used for capturing prey.

Fig. 14. Errant polychaetes:

a – oceanic polychaete Tomopteris; b – predatory polychaete Vanadis minuta capturing prey; c – Lepidonotus

Benthic forms inhabiting the sea bottom feature typical biramous parapodia with long setae primarily adapted for crawling. Among these, special mention should be made of members of the family Nereidae from the order Nereimorpha, which are distributed across all the world's seas. In northern seas, *Nereis pelagica*, reaching up to 20 cm in length (see Fig. 2), finds shelter among sea-wrack beds alongside the larger (up to 40 cm) *N. virens*, whose green body shimmers with iridescent colors. Several species of this genus inhabit the Black and Azov seas (*N. cultifera*, *N. succinea*, *N. diversicolor*).

Nereids inhabit burrows excavated in mud and sand and act as polyphages, feeding on both PLANT AND ANIMAL matter. During the reproductive period, driven by movements of modified parapodia, they ascend to the water surface and swim freely while releasing their Gametes. Shortly afterward, vast numbers of larvae appear in the plankton, get dispersed by currents, and eventually settle on the bottom.

Polychaetes of the family Aphroditidae (order Phyllodocemorpha) are extremely widespread. Their bodies are flattened and relatively short, and the dorsal surface is covered by two rows of large scales (elytra) that protect and camouflage the animal. Females deposit their eggs beneath these scales and carry them there for some time. The eggs hatch into trochophore larvae, which subsequently transition to a pelagic lifestyle in the water column. Aphroditids are predatory animals capable of everting their long Pharynx, which bears four jaws at the tip used to capture prey (polychaetes, Mollusks, etc.). In northern seas, *Harmothoe* and *Lepidonotus* (Fig. 14, c) are frequently encountered, as is the sea mouse (*Aphrodita aculeata*), a fairly large (about 10 cm) predatory worm covered with long, iridescent setae.

Benthic forms also include numerous species of the order Eunicemorpha. Among them, the palolo worm, mentioned above (see Fig. 10, b), is particularly well-known. This group also includes members of the families Protodrillidae and Polygordiidae, which exhibit a simpler Anatomical Structure than other polychaetes. These are relatively small worms (ranging from 1 mm to 8 cm in length) with bodies consisting of several dozen segments. They inhabit the coastal zone or shallow waters (up to 20 m deep) in sandy sediments, moving through the interstitial spaces between sand grains. These worms lack true parapodia; instead, they bear either tufts of setae (*Saccocirrus*) or ciliary bands (*Protodrilus*). *Polygordius* (see Fig. 11) lacks both cilia and setae, moving exclusively via the contraction of longitudinal Muscles. Some researchers place these families, along with dinophilids, into a separate class, Archiannelida, emphasizing the primitive nature of their bodily Organization. However, recent detailed studies suggest it is more likely that the organization of these worms has undergone secondary simplification as an adaptation to interstitial life in sandy substrates. This hypothesis is supported by the complex structure of their Brain, the presence of a segmented coelom, a well-developed Circulatory system, and specific developmental features.