Adaptive Radiation: Evolution and spread from a single ancestral species to a variety of forms occupying
different niches.
Habit and Habitat:
| Crawling | Live under stones, submerged vegetation. Well developed head bearing several kinds of sensory organs like eyes, paps, antennae and cirri. Parapodia are large and used in crawling or swimming. Body segments are similar. e.g. Nereis, Aphrodite |
| Planktonic and Pelagic | Adapted to live in open sea and are transparent. Tomopteris has large parapodia, but without setae. Longitudinal muscles are better developed than circular muscles. Septa are reduced in crawling and swimming species |
| Burrowing | Prostomium is reduced or absent. Eyes, paps and tentacles are usually absent. Parapodia are reduced. Circular muscles are well developed. Coelomic fluid has a skeletal role in locomotion. Move through substratum by peristaltic contractions. Setae are modified into hooks (Uncini) that help in gripping the wall of the burrow) e.g. Glycera |
| Tubicolous | Tubes secreted by them or built from gathered materials. Adaptations depends on the type of tube. Mucus lined burrows: adaptations similar to crawling polychaetes (Perinereis). Sand grain tubes: truncate head and parapodia reduced to ridges (Owenia). Tube made from sand cemented using mucus. Parchment tubes Calcareous tubes: Two large glands beneath the collar fold secrete the calcium carbonate added as rings to construct the tube. |
Types (modes) of nutrition:
| Carnivores | Most crawling, some burrowing and all pelagic polychaetes. Feed on small invertebrates including other polychaetes. Captured by means of Pharynx or proboscis. Pharynx have structures like chitinous jaws and teeth in various arrangements (depending on species). |
| Sand and mud detritus feeders | Bottom sand has bacteria, diatoms, microbes and dead organic matter. Direct deposit feeders: organic matter is digested, sand is egested as castings. Include burrowing and tube dwelling species. Indirect deposit feeders: lack a proboscis. Use ciliated grooved tentacles, secreting mucus. Food particles accumulate at the base of the tentacles and are conveyed to mouth. |
| Filter feeders | Proboscis is lacking. Many sedentary and tubicolous polychaetes. Head has bipinnate filaments or tentacles (radides) with a ciliated groove running along the oral surface. Used to collect food particles suspended in water. |