Phylum Annelida Flashcards
Annelida
• vermiform (worm-shaped) –elongate (longer than wide) –circular in cross-section (roughly) – soft-bodied • chitinous bristles (at least one pair) – setae/chaetae (apomorphy) • metamerism (metameric segmentation) –body divided into numerous repeating units –each unit repeats: (“skin”, muscles, nervous system, reproductive system, excretory system)
Characteristics
•body consists of: –complete regionalized digestive tract (tube within a tube) –body cavity - eucoelom - schizocoelous –12,400 described species –some of largest worm-like invertebrates •3 m earthworm - Australia •2 m earthworm - Africa –some very small •interstitial species (between soil particles) - few tenths of mm
Habitats
•virtually all habitats with sufficient water –many marine species – freshwater – terrestrial •free-living •commensal (with other species) •parasitic
Classification
• three major groups
• Class Polychaeta - marine segmented worms
- majority of annelids
• Class Clitellata
–Subclass Oligochaeta - earthworms and relatives -freshwater and terrestrial
–Subclass Hirudinea - leeches
- freshwater and terrestrial; few marine
Segmentation
•anterior prostomium
•peristomium directly behind
•posterior pygidium
•present before segmentation begins (presegmental)
•segments between peristomium and pygidium
produced during development and growth
Body wall
•outer layer –thin, flexible, permeable –cuticle secreted by epidermis (often) •thin layer of connective tissue •muscles: –circular (not always continuous) –longitudinal (sometimes in bands) •peritoneum (lines coelom)
Body cavity
- segmentally separated by septa (from peritoneum)
- compartmentalized coelom
- ancestral condition (retained in some)
- coelomic fluid in each segment isolated (allows independent movement of segments)
- polychaetes - cavity divided by septa and by digestive tract + peritoneum in centre
Metamerism
• body divided into “segments” or “metameres”
• evident externally as:
–rings or “annuli”
–repeated structures (e.g. appendages)
• internally - septa
–peritoneum from each segment with connective tissue in between
Serial homology
–repetition of structure
–different areas modified differently
• appendages
• internal structures
Nervous system
•anterior cluster of ganglia •longitudinal cords run posteriorly •cerebral ganglion (“brain”)
–connectives (nerves) around digestive tract to:
•suboesophageal (below digestive tract) ganglion
• pair of solid ventral nerve cords
• segmental ganglia (nerve cluster in each segment)
• may fuse to form single cord
• segmental ganglia may fuse
• giant nerve fibres in cord
Circulatory system
•closed circulatory system
•fluid enclosed in vessels
–leeches different
•major dorsal vessel - anterior transport
•major ventral vessel - posterior transport •capillaries connect two on each side
• no “heart” - specialized pumping organ
• vessels have intrinsic contractile properties
• contraction + valves –> one-way circulation
Pigments
–hemoglobin (oligochaetes, leeches)
• in cells and dissolved
–chlorocruorin (iron, but green - dissolved)
–hemoerythrin - iron - in cells
Excretory system
• Protonephridium –“end cell” closed, filters material –works in small or acoelomate organisms –usually many, so close to fluids • Metanephridium –open funnel –larger animals, open body cavity/more fluid
Metanephridium
–open tubes - coelomic fluid flows to outside
–drawn into tube by cilia
–tube wrapped in circulatory vessels
–absorption and excretion along length of tubule (like human)
–excreted fluid very different from: • coelomic fluid
• circulatory fluid
–role in osmoregulation
–funnel opening (nephrostome) in one segment
–rest of unit (pore to outside (nephridiopore), tubule and blood vessels) in next segment
Digestive tract
Complete
Not metameric
