1: Annelids Flashcards
What are annelids known as?
Segmented worms
The annelids are coelomates, describe this.
They have a body cavity (coelom) between their ectoderm (surface) and mesoderm
What are the advantages of having a coelom?
Transport
Gut moves independently of body wall
Site for gamete maturation
Hydrostatic skeleton
Describe the hydrostatic skeleton of the Annelids
- Water incompressible → base against which muscles can contract
- Circular muscles - when contracted worm is long & thin
- Longitudinal muscles - when is contracted worm is short & fat
Antagonistic muscles
Describe how the Annelids move
Peristalsis → Waves of contraction by longitudinal/ circular muscles
Metameric segmentation makes this more efficent and precise
What sort of segmentation is found in Annelids?
Metameric: the body is divided into interconnected segments
What is the ‘head end’ known as in Annelids?
Prostomium
What is the ‘tail end’ known as in Annelids?
Pygidium
Where are new body segments formed in Annelids?
Proliferation zone
What structure divides the segments in Annelids?
The septum
Describe the excretory system in annelids
Metanephridium:
Fluid forced out of blood vessels by muscular contraction, it floats around in the coelom
Goes through metanephridium
Useful substances reabsorbed
Waste passed out through gut
Describe the Polychaeta
- Largest group within the annelids
- Predominantly marine worms
Describe the general body plan of the polychaetes
- Parapodia → fleshy projections, have various functions
- Paradodia covered in lots of setae → hairy things
- Prostomium well developed:
In predacious form → protruding jaw, eyes, nuchal organs (sense chemical cues)
Give an example of an errant form of Polychaeta
Nereis (ragworm):
Well developed parapodia
Predacious head
Give an example of a sedentary form of Polychaeta
Arenicola (lugworm):
Burrowers
Reduced parapodia
Feed on mud, extract organic material and excrete waste
Live in U shaped burrow, draw sand in, back up burrow = worm cast
Describe Polychaete reproduction
- Mostly dioecious (separate sexes)
- External fertilisation (due to marine environ)
- Larval stage (trochophore) → like molluscs
- Spawning may be synchronous
- Body transformed by epitoky into reproductive form.
E.g Males form elaborate parapodia for swimming around
E.g Epitokes (packets of gametes) form at the end of the worm and bud off
Very generally describe the Clitellata (oligochaetes & hirudineans)
No parapodia
Produce clitellum & cocoons
No larval stage!
Hermaphrodite
Gonads restricted to a few segments
What are the Oligochaetes known as?
Earthworms
Describe the Oligochaetes
- Peristaltic locomotion = well developed septa between segments
- Gas exchange over body surface → must be damp, can produce mucus if terrestrial
What is aestivation in Earth worms?
Dry environ triggers a stress response called aestivation (resting stage)
Describe reproduction in the Oligochaetes
Hermaphrodites: mutual exchange of sperm
Clitellum important for repro
1. Earthworms pair up head to tail
2. Clitellum of each worm produces mucus and binds worms together
3. Male gonopore produces sperm which travels down to the other worms spermathecal opening
4. 2 worms separate, mucus formed by clitellum forms the cocoon at top of head
5. Within cocoon fertilisation takes place, mini worms hatch out (no larval stage)
Describe the Hirudineans
- No setae, dorsoventrally flattened
- Restricted no. of segments (34)
- Suckers
What are the Hirudineans also known as?
Leeches
Describe repro and parental care in the Hirudineans
- Mutual sperm transfer
- Sometimes hypodermic impregnation
- Produce cocoons which develop into mini leeches
- Some brood young and show parental care
- Some even have a pouch that they keep their young in
Describe locomotion in the Hirudineans
No septa, crawl with suckers
They have lost the septa (earthworms) that allows peristalsis
Still get waves of muscular contractions though
Describe the predacious nature of the Hirudineans
- Proboscis/ jaws
- Some feed on small inverts:
Swallowed whole
Suck out hemolymph - Most regarded as parasitic: blood suckers latch onto vertebrates
- Produce coagulant → hirudin
- Can take up to about 5-10x body weight in blood
- Symbiotic bacteria in gut to aid digestion
- Once they have a good meal they don’t need to feed for another 6 months!
Compare the protonephridia with the metanephridia
- Protonephridia have a single opening to the outside environment.
- Metanephridia have two openings, one to the coelomic cavity and the other to the outside
- Proto considered more primitive
- Protonephridia consist of flame cells and tubules.
Metanephridia consist of tubules and ciliated funnels
