ANNELIDS Flashcards
key characteristics
(a) Bilaterally symmetrical worms.
(b) Body consists of segments added to the back end of a
trochophore larva.
(c) . Through gut
(d) . Coelomic spaces extensive, often important in locomotion
(e) . Usually a separate closed circulatory system
(f) . Well developed nervous system, with an anterior brain and paired nerve cords with a ganglion in each segment
(g) . Often hermaphrodite, and some also reproduce asexually
2 annelid classes
Polychaeta and Clitellata
Distinguish between polychaetes and clitellata
Polychaetes: with well-developed limb-like parapodia tipped with bristles (chaetae) on each segment, and pronounced regional specialisation of the body. Mainly marine. (RAGWORMS)
Clitellata: segments bear only chaetae, without parapodia.
No external regional specialisation except for a specialised reproductive region (the clitellum). Mainly fresh water or terrestrial. (EARTHWORMS AND LEECHES
Clitellata are divided into 2 sub classes
Oligochaeata
Hirudinoidea
- Leeches. No chaetae or parapodia, usually with anterior and posterior suckers, predatory or parasitic. Freshwater, marine and terrestria
What is segmentation?
Repetition along body of entire units,
What units are segmented?
- A coelomic space separated from neighbouring
segments by septa in front and behind. - circular and longitudinal sets of muscles.
- a ganglion along the paired ventral nerve cord.
• a pair of appendages - parapodia and chaetae in
polychaetes, only chaetae in oligochaetes, and
nothing in leeches.
• a pair of coelomoducts modified into excretory organs
called metanephridia. These consist of a ciliated
funnel inside the coelomic space, and a duct to the
outside.
• Blood vessels, in a closed circulatory system, linking
the major dorsal and ventral vessels.
Why be segmented?
1) in order to have local control over shape changes
2) hydrostatic skeleton
3) essential for peristaltic burrowing
4) for tagmatisation (parapodia for walking/ swimming in ragworms or Poison-filled hollow spines in the fireworm Hermodice)
- Reproduction and development.
Segmental body plans allow growth by repeated expressions of a relatively simple set of developmental genes, whose impacts can then be modified to produce regional differentiation. Segmental bodies are also relatively easy to reproduce asexually by budding, or to repair when parts are lost
Describe peristaltic burrowing
- relies on the fact that the coelomic fluid is incompressible, termed a hydrostat
- contraction and relaxation of longitudinal and circular muscles at opposing times
- chetae act as ratchets (preventing backwards movement)
Other than peristaltic burrowing, describe the other key role of hydrostatics in predatory polychaetes
Used for an eversible pharynx
Distinguish between errant and sedentary
SEDENTARY
- live a burrowing lifestyle
- anteror segments are specialised for food collection, gas exchange and tube building
ERRANT
- predatory and free living, even pelagic
- Errant species often possess long palps for prey detection, an eversible proboscis and chitinous jaws.
Where do new segments arise from in development?
growth zone in front of pygidium (tail)
Terrestrial adaptations eg. leeches
- aestivation
- internal fertilisation
- shelled eggs
- suppression of larval development
- gas exchange by compete blood supply to skin
Well developed reproductive region in Clitellata is called (a). These live in (b) or are terrestrial.
freshwater
In peristaltic burrowing, when the longitudinal muscles (a), the circular muscles (b). Segments are widened to give (c) via the (d).
a) relax
b) contract
c) grip
d) chetae
What is a ratchet?
Which part of body acts as a ratchet?
- prevents backwards movement
- chetae
