General Characteristics
- The largest annelid class
- Mostly marine
- Head with eyes and tentacles
- Parapodia bear numerous setae
- Monoecious or dioecious
- Development frequently involves a trochophore larval stage
- Live on ocean floor, under rocks and shells, and within crevices on coral reefs
- Some are burrowers
- Some construct tubes of cemented sand grains or secreted organic materials
- About 5,300 species
- Example: Nereis, Arenicola, Sabella
External Structure and Locomotion
- Parapodia- lateral extensions; chitinous rods support the parapodia and numerous setae project from the parapodia.
- Setae (also called cheatae)- bristles secreted from invaginations of the distal ends of parapodia; aid in locomotion by digging into the substrate and also hold a worm in its burrow or tube
- Prostomium- a lobe that project dorsally and anteriorly to the mouth and contains numerous sensory structures including eyes, antenae, palps, and ciliated pits or grooves called nuchal organs
- Peristomium- first body segment; surrounds the mouth and bears sensory tentacles or cirri
- Cuticle- the epidermis of polychaetes consists of a single layer of columnar cells that secrete a protective non living cuticle.
- Capable of walking, fast crawling, or swimming.
- The longitudinal muscles on one side of the body act antagonistically to the longitudinal muscles on the other side to make undulatory waves from the posterior end toward the head
- make burrows by pushing through the sand or eating the sand