Annelids (Segmented Worms)

Phylum Annelida (Segmented Worms)

• Segmented worms
• Annulus = "ring"
• Basic characters
• Segmented
• repeating units separated by septa: thin sheet of tissue
• Eucoelomate: true coelomates
• Closed circulatory system
• Blood vessels
• Complete digestive system with anus

Class Polychaeta

• Contains most annelids
• Mostly marine
• Have parapodia = "sort-of feet"
• Each bears many chaetae: bristles
• Differentiated head
• Sensory antennae
• Jaws
• Eyes
• More developed in errant species than sedentary species
• Errant: very active organsim
• Sedentary: not very active organism

Class Polychaeta (Continued)

• Predatory
• Feeding
• Ingest substrate to digest organic matter
• Suspension feeders: filter food out of water column
• Deposit feeders: collect food with sticky tentacles
• Sedentary Polychaetes dig holes and construct tubes for protection
• Free spawning
• Release gametes into water
• External fertilization
• Larvae are planktonic trochophores
• Trochophores: wheel like arrangement of cilia
• Benthic: live in the lowest part of the water

Class Oligochaeta

• Less diverse morphology and ecology
• Lack parapodia and developed heads
• Non-leech species retain some parapodia
• Terrestrial and freshwater
• Feeding
• Ingest substrate to digest organic mater
• Not free spawning
• Hermaphrodites: simultaneously male and female
• Transfer gametes directly into female organ
• Copulation: transfers sperm into others seminal receptacle (and vice-versa)
• Clitellum: secretes gel-like substance to form cocoon
• Raised band that is thicker than the body
• Deposit eggs into cocoon
• slides cocoon forward where sperm will be deposited
• External fertilization
• Direct development: young are hatched looking like adults
• Terrestrial Oligochaeta
• Important for soil formation
• Breaks down organic matter
• Aerates soil
• Composting

Subclass Hirudinae

• Leeches
• Mostly freshwater
• Has clitellum
• Lacks parapodia and chaetae
• Predators
• Prey on soft-bodied inverts
• Some feed on blood
• Use anesthetics and anticoagulants to suck blood without detection
• Used to be used in medicine
• Helps blood travel to re-attached extremitie