Annelids Flashcards
basic features?
~17000 marine, terrestrial and freshwater free living, symbiotic, parasitic soft bodied with bilaterial symmetry thin body wall with moist surface for gas exchange
body plan/features?
two part head - protsomium with cerebral ganglion and sensory organs
peristomium - mouth
series of segments - metamers
terminal pygidium - contains anus (this is where the new segments form)
paired chitinous setae (bristles)
gut is a straight tube, not segmented
what is metamerism?
body contains a series of repeating segments
segments separated by septa - thin mesoderm tissue
coelomic fluid acts as a hydrostatic skeleton
circular and longitudinal muscles act against coelomic fluid
contraction of longitudinal = short and fat
contraction of circular = long and thin
each segment controlled independently
how do annelids burrow?
intersegmented septa perforated or lost
- circular muscles in posterior part of body forcing coelomic fluid forward
- anterior segment swells and anchors the body
- posterior longitudinal muscles contract pulling the back of the worm forward
- thrust the proboscis forward to deepen the burrow
excretion in annelids?
nephridia - tubular excretory organs
most segments contain two nephridia - each opens at both ends
coelomic fluid drawn into the nephridium at the nephrostome by ciliary action, excretion of of metabolic waste through nephridopore
circulatory system in annelids?
closed
DV longitudinal vessels - perforate septa, blood flow anteriorly in dorsal vessel, flows posteriorly in ventral vessel
connected by segmented pairs of vessels
peristalsis maintains blood flow
some species have vessels that expand/contract to pump blood
nervous system in annelids?
dorsal cerebral ganglion - in protomium
one of more ventral longitudinal nerve cord
high sensitivity to touch - touch receptors distributed over body surface
statocysts - help maintain proper orientation when burrowing
photoreceptors - direct light direction and intensity - shadow response where the sudden decrease in light intensity
some have photoreceptors along length
how has the phylogeny changed?
relationships hard to discern
phylogeny changes frequently
traditionall divided into three - polychaeta, oligochaeta and hirudinea
closest relative of hirudinea is a group of oligochaetes so they form a monophyletic group - clitellata (characterised by clitellum)
recent phylogeny supports two classes
- errantia and sedentaria
features of errantia?
marine - benthic and pelagic
well developed head - tentacles, sensory palps, eyes, antennae
paired appendages - parapodia, many setae
what are parapodia?
thin, flattened paddle like flaps
highly vascularised - function as gills
numerous setae
aids in locomotion, hold burrowing species in there burrow
how do annelids move?
longitudinal muscles contract on one side, relax on the other
undulatory waves
parapodia and setae push against the substrate
- extend when longitudinal muscles relaxed
- contract when longitudinal muscles contracted
how do annelids reproduce?
sexually immature - atoke
sexually mature - epitoke
separate sexes with external fertilisation - muscles in posterior segments degenerate becoming swollen with gametes, anterior portion stays in sand and generates new segments
at surface the epitokes burst and releasing eggs and sperm - mass spawning
structures for reproduction?
gonads are temporary structures, they shed gametes into the coelom which then leave the body through gonoducts, nephridium or by the rupture of the body wall
how do annelids feed?
predators or scavengers
muscular pharynx with jaws that are thrust out at great speed when contraction in the anterior segments increases hydrostatic pressure in coelomic spaces causing an aversion of the pharynx - jaws gape when pharynx everted - once prey in jaws the coelomic pressure released
pharynx and prey pulled into body by large retractor muscles
