circulatory system blood vessels (closed circulatory system)
digestive tract with separate mouth and anus
most have setae – bristles of chitin used to anchor the worm to a substrate (“bristleworms”)
closed circulatory system has some enlarged vessels that serve as hearts
gases are exchanged at skin (no gills or lungs)
excretory system includes nephridia (like in mollusks)
2 per segment
transport waste out of coelom by excretory tubules
3 classes: Polychaeta (polychaetes), Oligochaeta (earthworms), and Hirudinea (leeches)
Class Polychaeta (grade) – polychaetes
many unusual and colorful forms; include plumed worms, peacock worms, fan worms, and many others
~10,000 living species, mostly marine
often live in burrows
usually filter-feeders, sometimes carnivores or parasites
distinctive characteristics:
typically more cephalized than other annelids
parapodia- paired, fleshy, paddle-like flaps on most segments- used for moving and gas exchange
sexes separate, but usually lack permanent gonads (sex organs); instead, make gametes from cells lining coelom or septa
external fertilization
trochophore larvae – similar to that of mollusks (ciliated)
Class (or subclass) Oligochaeta (grade) – earthworms
mostly terrestrial (in moist soil); a few aquatic species
eat their way through soil (usually eat own weight in soil each day; most soil has passed many times through worm guts)
100-175 segments, with mouth on first one and anus on last one
no eyes, but some light-sensitive organs near end of body
reproduction
hermaphroditic
join in opposite directions at the clitellum (obvious thickened band), which secretes mucus that holds the pair together during copulation
exchange sperm, and then each lays eggs in mucous cocoon surrounded by chitin that is secreted by the clitellum (this cocoon protects the fertilized eggs)
Class (or subclass) Hirudinea (clade) – leeches
mostly freshwater species; some marine, some terrestrial
hermaphroditic with clitellum (only during breeding season)
reduced segmentation (34 segments), and coelom is reduced and continuous (septae lost)
usually dorsoventrally flattened (resemble flatworms)
all but one species have no setae
have suckers at one or both ends of body
includes parasites, predators, and scavengers
many suck blood (external blood-sucking parasites)