Annelida: the (mostly) segmented worms
Dr Natalie Welden
By the end of this lecture
By the end of this lecture
- Describe and recognise the main characteristics of the annelida
- Describe the main classes and list their general features
- Understand their main biological traits including feeding, reproduction and locomotion
Diagnostic Characteristics
Bilaterally symmetrical
Spiral cleavage
Diagnostic Characteristics
Diagnostic Characteristics
- Closed cirulatory system
- Diffusion + specialised structures (parapodium + gills)
- Respiratory pigements
- usually hemoglobin or chlorocruorin
Diagnostic Characteristics
- Well developed nervous system
- ganglionated nerve chords
Diagnostic Characteristics
- Segmentarilarly arranged chaeta
Diagnostic Characteristics
- Jaws in Eunicidae and Phyllodocidae
Diagnostic Characteristics
- Serial repetition of segments
- teloblasts, stem cells that are responsible for sequential segment addition
- Evolved three times
- Balavoine (2014)
Diagnostic Characteristics
Diagnostic Characteristics
At what stage do segements develop?
Balavoine (2014)
Diagnostic Characteristics
- Gonochistic/Hermaphoroditic
- Gametes form in the coelom and are released via flame cells, gonoducts or damage to the meso/ectoderm
- Epitoke - reproductive segment
- Both direct and indirect development
- Some have trocophore larvae
Classification of Annelids
Unsegmented annelids? Possible origins of four lophotrochozoan worm taxa.
"Recent analyses suggest that annelids are much more diverse than traditionally believed, and that polychaetes are paraphyletic.
Some lesser-known taxa (previously regarded as separate phyla) appear to fall within the annelid radiation."
Polychaeta
- Blood system uses haemoglobin or related compounds
- Divide into ecological groups: errant and sedentary species
Polychaeta: Errant Species
- Gas exchange associated with parapodia
Polychaeta: Errant Species
Polychaeta: Sedentary Species
- Live in permanent burrows or tubes
- Specialised feeding structures on head
- In fan worms: prostomium develops
Polychaeta: Sedentary Species
Lamellibranchians
Chemoautotrophs
- Provide bacteria with hydrogen sulfide and oxygen by taking them up from the environment and binding them to a specialized hemoglobin molecule.
Polychaeta: Sedentary Species
Polychaeta: Errant Species
Indirect development in Platynereis dumerilii
Polychaeta: Errant Species
Clitellata
- Species possessing a clitellum 'Collar'
- Around 8'000 species across marine, freshwater and terrestrial classess
- 3 major subclassess
- Oligochaeta
- Hirudinea
- Branchiobdellae
Oligochaeta - "few setae"
Around 10'000 known species
Three orders
Longest 3 meters
Few sensory organs
Excretion via metanephridia
Use haemoglobin for oxygen transport
Oligochaeta
Earthworms are extremely important and play a vital role in recycling plant nutrients and aerating the soil.
27 species of earthworm in the UK
Hirudinea
Around 700 species - with approximately 500 freshwater species
Move by looping
Predatory and parasitic
Use anticoagulant enzymes
Branchiobdellida
Leech like ectoparasites of crayfish
Around 100 species
Anterior and posterior suckers
Echiura
Around 230 species
2 orders
All marine, most burrowers
Scoop shaped probocis
No segments
No organ repetition
Open circulatory system
Sipuncula - "little siphon"
Around 300 species
3 Classes - 4 orders
All marine
Up to 10 cm
Mouth surrounded by cilliated tentacles
No segments
No organ repetition
Summary...
- Segmentation
- Movement
- Coelomic cavity (formation of)
- Larval stages
- References
- Halanych K. (2004). The new view of animal phylogeny. Annu Rev Ecol Evol Syst 35: 229-256.
- Casey W. Dunn et al (2008) Broad phylogenomic sampling improves resolution of the animal tree of life. Nature 452: 745-749.
- And many more…