Annelida classification Flashcards
What are annelida
Segmented Lophotrocozoa with coelom
Characteristics of the phylum Annelida
Large phylum, 16,500 species
<1mm to ~3m (Australian Earthworm)
Marine, fresh water and terrestrial
Suspension feeders, active predators, parasites, detrivores
Enormous ecological & economic importance
Re-emergence in medical use
Most distinctive feature is metamerism = division of the body into similar parts or segments that are arranged in linear series.
Annelida classification
Traditionally divided into 7 classes and subclasses
Phylogenetic analysis indicates most of these are either paraphyletic or polyphyletic
Polychaetes
Many chaetae (lots of hair), Chaetae are bristles or setae made out of chitin
Largest class - 10,000 species but probably paraphyletic
Most r 5cm-10cm
Highly varied lifestyles, tube welling, burrowing, pelagic
Significant in marine food webs
Generalised body plan
of polychaetes
Segmented worms
Metameric (repeated) structures are confined to the trunk
The protisomium (head), the peristomium is behind the protisomium, and pygidium (terminal part) arent segments, new segments added just before pygidium
Well differentiated head with specialised sense organs. Chitinous jaws (light and strong)
Prostomium: bears eyes, antennae & palms sometimes retractile
Polychaetes - Biramous parapodia
Polychaetes have characteristic bigamous parapodia. These are paired, fleshy appendages that extend from the body segments. The terminal end bears setae. Parapodia and setae facilitate movement and respiration, some have defensive role (eg neurotoxins in fireworms)
Biramous parapoda can be used to facilitate rapid locomotion
Polychaetes - Brain
Brain lies in the prostomium. Brain supplies nerves to the palms, antennae and eye. Ventral nerve cord is double through the segments, although the two chords are often fused. Rapid contraction to avoid predators aided by giant axons. Substantial selection pressure to protect soft body
Polychaetes - sense organs
Pigment cup eye; detects light intensity and source
Nuchal organs on head; chemoreceptors
Statocytes found in burrowers; detect changes in orientation
Polychaetes - Reproduction
Asexual reproduction occurs
Sexual reproduction is most common
Most are gonochoristic
There are no permanent sex organs
When the worm is mature it is packed with sperm or eggs
Fertilization is external
Epitokes (pelagic morphs capable of sexual reproduction swarm and swim to the surface of the ocean). At sunrise they rupture and fertilization follows
Polychaetes ecological importance
Theyoccupy vertical or U shaped burrows.
Polychaetes are ecologically important because Serpulids secrete calcareous tubes which stabilizes muddy habitat
Clitella-animals with a clitellum
Oligocheata
Earthworms and close relatives
Cl = clitellum (makes mucus for mating; secretes egg sac)
Hirudinea (Leeches)
Co = egg cocoon produced by clitellum
Earthworm - reproduction
Hermaphrodites (have both male and femail reproductive organs)
During copulation there is mutual sperm transfer
Clitellum secretes mucus coat to hold worms in position
The clitellum also secretes a chitin-like material to form a cocoon
Fertilisation within cocoon
Direct development (no trochophore larva)
Earthworm - Hydraulic skeleton & Locomotion
The coelomic fluid functions as a hydraulic skeleton. There are 2 sets of muscles; longitudinal and circular muscles. Widening and elongation can be restricted to certain segments. Waves of peristaltic contraction pass down the length of the body and cause elongation and shortening in each segment. Chitinous, paired, lateral bristles increase traction with the substratum
Earthworm - how is the body wall and coelom similar or different from other Annelida
The body wall is similar to the burrowing polychaetes
Compared to polychaetes, earthworms have the best developed septa – very efficient burrowing
They have sphincters around septa to control the flow of fluid
Oligochaeta earthworms & close relatives
~3000 species
Terrestrial species, live in burrows, branched interconnecting tunnels
Earthworms essential to functioning of many terrestrial ecosystems- but can also be damaging invasives
Earthworms - How do they differ from other Annelida
- Less differentiation of head region
- Reduced sense organs
- No parapodia
- Clitellum (visible in mature animals)
- Each segment 4 pairs chaetae (most earthworms)
- Hermaphrodites
Hirudinea (Leeches)
- ~500 species
- Predominately fresh water, some moist terrestrial habitats
- most 2-6cm, up to 30cm
- Many predators
- Some parasitic (temporary or permanent)
- Larger than polychaetes
Hirudinea (Leeches) - Distinctive traits
- Fixed number of segments
- Anterior & Posterior suckers
- No parapodia, chaetae rarely
- Reduced metamerism (which is..?)
- Testes, ovaries & some ganglia in repeated units
- Coelom used as continuous hemal system (no septa)
Hirudinea (Leeches) - Locomotion
- Leeches crawl but only the anterior and posterior suckers anchor to the substratum
- When the Posterior suckers is anchor, a wave of circular contraction sweeps over the body and the body is lengthened and extended forward
- Diagonal muscles maintain hydrostatic pressure and keeps the animal rigid while it is raised and attached posteriorly
- The anterior sucker then attaches and the posterior sucker releases
- A wave of longitudinal contraction occurs, shortening the animal and moving the posterior sucker forward
Hirudinea (Leeches) - Medicinal uses
“Blood letting”
local anaesthesia, anti-inflammatory
Promote blood flow & healing
Saliva contains at least 60 proteins
Anti-coagulant hirudin is also purified & used to treat blood clots
Most leeches predatory on other invertebrates,
what are the challenges of this life style?
- Finding hosts (senses)
- Accessing blood
- Evading host defences (anaesthetics)
- Keeping the host’s blood flowing
