INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM METAZOA

SUBKINGDOM EUMETAZOA

SECTION TRIPLOBLASTICA, OR BILATERIA

SUBSECTION SPIRALIA

PHYLUM ANNELIDA

SUBPHYLUM ACLITELLATA

CLASS POLYCHAETA

SUBCLASS ERRANTIA

This subclass comprises predominantly free-living predatory polychaetes, or more rarely, detritivores. They possess well-developed Sense Organs, mostly homonomous trunk segmentation, parapodia along the entire body, and often well-developed gills. Most errant polychaetes live on the bottom surface, although some species form mucous tubes or burrow into the substrate; swimming (pelagic) forms also occur.

Pelagic species are predominantly predators with robust jaws in the buccal region. Their parapodia lack setae and are modified into peculiar swimming fins. Benthic forms possess typical biramous parapodia with long setae adapted for crawling. Among them, Representatives of the family Nereidae are the most widespread, occurring in all seas of the world. In northern seas, common species include Nereis pelagica, 20 cm in length (Fig. 166), and the larger (up to 40 cm) N. virens, whose body is colored in shades of green and shimmers with all the colors of the rainbow. Several species of this genus inhabit the Black and Azov seas (N. cultrifera, N. succinea, N. diversicolor). Nereids dwell in burrows they dig in mud and sand; they are polyphagous and feed on both PLANT AND ANIMAL matter. One tropical species of the genus Lycastopsis lives on banana and cacao plantations, feeding on decaying leaves and sweet fruits.

Benthic polychaetes also include species of the family Aphroditidae. These are predatory animals with a short, flattened body bearing two rows of large scales on the dorsal side. Females deposit eggs under these scales and carry them there until the larvae hatch. In northern seas, Harmothoe and Lepidonotus (Fig. 175) are frequently encountered, along with the relatively large (about 10 cm) predatory worm—the sea mouse (Aphrodite aculeata)—which is covered with long, iridescent setae. Benthic forms also encompass numerous species of the order Eunicemorpha, among which the most well-known is the palolo worm mentioned above (Fig. 171). This group further includes representatives of the families Protodrillidae and Polygordiidae, which have a simpler Structure than other polychaetes. These are relatively small worms (from 1 mm to 8 cm); instead of parapodia, they bear either tufts of setae (Saccocirrus) or rings of cilia (Protodrilus), whereas Polygordius (Fig. 172, c) lacks both cilia and setae. They inhabit shallow waters in sandy bottoms, moving between sand grains.

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Fig. 175. Errant polychaete Lepidonotus (from Hayevsky)