Annelida

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

• Eucoelomate

• Complete digestive tract

Diagnostic Characteristics

• Closed cirulatory system
• Diffusion + specialised structures (parapodium + gills)

• Respiratory pigements
• usually hemoglobin or chlorocruorin

Diagnostic Characteristics

• Well developed nervous system

• dorsal cerebral ganglion

• ganglionated nerve chords

Diagnostic Characteristics

• Segmentarilarly arranged chaeta

Diagnostic Characteristics

• Prostomium

• Peristomium

• 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

Remember - teleoblasts!

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.

Halanych et al (2002)

"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

• More than 12'000 species

• From the Greek meaning

• Blood system uses haemoglobin or related compounds

• Divide into ecological groups: errant and sedentary species

Polychaeta: Errant Species

• Prominent sense organs

• Gas exchange associated with parapodia

Polychaeta: Errant Species

Evertable pharynx

Polychaeta: Sedentary Species

• Live in permanent burrows or tubes

• Parapodia reduced

• Specialised feeding structures on head

• In fan worms: prostomium develops

Eunice aphroditois

Polychaeta: Sedentary Species

Lamellibranchians

Chemoautotrophs

• Bacterial Symbionts

• 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

Presettlement forms

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

Movement via peristalsis

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

The spoon worms

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

The peanut worms

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…