Annelida Flashcards
Phylum Annelida - Characteristics
About 15,000 species
Segmented worms living in marine, freshwater, and moist terrestrial habitats
2/3 are the more obscure marine worms.
Metamerism
Segmentation/Metamerism – body divided into a serial succession of repeated units (segments, metameres or somites). Significance?
independently evolved: lophotrochozoans - annelids ecdysosomes
deuterostomes
greater complexity in structure and function
Burrowing efficiency
Fine control –nervous system
Redundancy –safety
Annuli - external circular grooves delimiting body segments
Septa (partition) – internal delimitation of body segments
Annelid Characteristics
Head –sensory+feeding. Typical 2 part annelid head:
Prostomium: anterior most region of the body, anterior to the mouth
Peristomium: pre-segmental region of the body surrounding the mouth.
Nervous System
Nervous system with double ventral nerve cord and a pair of ganglia with lateral nerves in each segment; brain a pair of dorsal cerebral ganglion with connections to ventral nerve cord
Sensory system
tactile organs, taste buds, statocysts (in some), photoreceptor cells, and eyes with lenses (in some)
Hydrostatic Skeleton
Hydrostatic skeleton - coelom filled with fluid (except for leeches)
Fluid volume constant
Contraction of longitudinal muscles shorten and expand the body
Contraction of circular muscles narrow and lengthen the body
By separating this force into sections, widening and elongation move the whole animal
Alternate waves of contraction, or peristalsis, allow efficient burrowing
Swimming annelids use undulatory movements
Circulatory System
Oligochaetes & Polychaetes have a closed circulatory system
dorsal vessels carry blood anteriorly
ventral vessels carry blood posteriorly
vascular valves ensure unidirectional blood flow
Heart
dorsal vessel is contractile
Aortic arches maintain steady pressure in ventral vessel
Excretion
Metanephridia: excretory organs of the adult annelid
- one pair of metanephridia found in each segment (except 1st 3 and last)
- metanephridia are highly vascularized with a capillary network
*nephrostome
- ciliated funnel that draws in coelomic fluid
- opens into segment anterior
to metanephridium
nephridiopore - terminal end of metanephridium - opens to the outside, eliminated waste - in some forms, nephridiopores are enteronephric
Class Polychaeta - Diversity
Largest class of annelids - >10,000 species,
mostly marine and usually benthic
some live in crevices, others inhabit tubes, or are pelagic
1 mm - 3 m long
Euryhaline - tolerate a wide range of salinity,
Warmer regions have more freshwater polychaetes
Paired appendages called parapodia on most segments
Many setae arranged in bundles on parapodia
No clitellum
Class Polychaeta - Characteristics
Well-differentiated head with sense organs
Prostomium retractile or not
Often bears eyes, tentacles, and sensory palps
Peristomium surrounds mouth and may have setae, palps, or chitinous jaws
Ciliary feeders may bear a tentacular crown that opens like a fan but can be withdrawn into the tube
Depending on the life style - 2 morphotypes:
Sedentary polychaetes mainly tube-living
Errant polychaetes may be free-moving, burrowing, or crawling
Clade Clitellata
Class Oligochaeta & Class Hirudinida
Earthworms & Leeches
Freshwaters, or terrestrial soils
Form reproductive structure called a clitellum
Ring of secretory cells found in a band around the body
Permanent in oligochaetes but visible only during reproductive season in leeches
NO parapodia
Hermaphroditic (monoecious) - with direct development
Young develop inside a cocoon secreted by the clitellum, and emerge as small worms
Class Oligochaeta
Diversity
Over 3000 species
Occur in habitats from soil to freshwater
Few are marine or parasitic
Nearly all bear setae
Fewer in number than in polychaetes
Form and Function
Earthworms - sometimes called “night crawlers”
Burrow in moist rich soil and usually live in branched interconnected tunnels
Oligochaeta Locomotion
cuticle, with pores for mucus secretion
epidermis, epithelial cells + mucous, sensory and photoreceptor cells
circular muscles (metamere elongation)
longitudinal muscles (metamere widening - shortening)
setae, paired chitinous “bristles” to increase traction typically ventro- and dorsolateral
Mechanism: waves of peristaltic contraction
Class Hirudinida: Leeches
- Diversity
Most freshwater, few marine or live in moist terrestrial environments
More common in the tropics
Vary in color: black, brown, red, and olive green
Most are flattened (dorsoventrally)
Feeding:
most carnivores feeding on small invertebrates
others - temporary or permanent parasites
Hermaphroditic
Form a clitellum during breeding season
Secretes a cocoon for reception of eggs
Leeches: Form and Function
Usually have a fixed number of segments
Appear to have more due to superficial annuli
NO distinct coelomic compartments
NO septa
In most, coelomic cavity filled with connective tissue and spaces (lacunae)
Lacunae channels may serve as auxiliary circulatory system
NO setae
Developed suckers for attachment
Most use suckers to attach and “inchworm” along surfaces
Closed circulatory system
Leeches - Feeding
Many are predaceous, some parasites,
Most are fluid feeders that prefer tissue fluids and blood pumped from open wounds
Freshwater leeches have a proboscis for ingesting small invertebrates as well as to suck blood
Bloodsucker leeches (medicinal leeches) have chitinous ‘jaws” and salivary glands (secrete an aesthetic and anticoagulant enzymes)
Gut specialized for storage of large quantities of blood
Some parasitic leeches leave their host during breeding season
Fish leeches may remain on a host
