BIOL 2003: Lesson 6 (Annelids)

Annelida

• ringed worms

  • setae form rings around the body, first instance of segmentation

• may be free living, symbiotic, parasitic

• closed circulatory system

• centralized nervous system

Metamerism

• body composed of serially repeated units seperated by septa (wall of tissue) 

• each unit contains components of most organ systems 

  • derived from mesoderm, therefore repeated muscles/organs 

• coelom plays large role 

  • fully closed, and filled with a fluid which allows it to serve as a hydrostatic skeleton

  • contraction of muscles allow for body to narrow/lengthen, as well as shorten/expand

Setae

• chitinous, epidermal bristles

• helps in burrowing, tubes, and locomotion 

• not seen in leeches 

Parapodia 

• paddle-like ‘feet’

• used in swimming, crawling, sensory, respiration, and potentially as modified gills 

• secondarily lost in many groups 

  • seen in mostly marine, motile worms

Errantia 

• motile, polychaete (meaning many long hairs) body plan

• mostly marine 

• well-developed sense organs and cephalization 

• external fertilization with trochophore larvae 

Sedentaria

• includes sedentary tube-dwelling polychaetes, beard worms, spoon worms, oligochaetes, and leeches 

• modified heads for particle or filter feeding 

  • or reduced heads for deposit feeding 

Sedentaria ‘polychaetes’

• tube dwellers

  • calcerous, mucus, sand grains 

• cilia, mucus, and food grooves on tentacles for food capture 

• reduced parapodia 

  • gill-like feet smaller than in errantia 

• reduced setae 

  • anchor into burrows and tubes 

Sedentaria ‘Siboglinidae’

• beard worms 

• secrete long, chitinous tube-dwellers 

• no mouth or digestive system 

  • absorption via chemoautotrophic bacteria 

Sedentaria ‘Echiuridae’

• spoon worms

  • spoon= flattened, extensible proboscis 

• secondarily unsegmented 

• homologous annelid setae 

  • having a common ancestor with the setae of annelid worms 

Clitellata

• subsection of sedentaria

• named after clitellum (reproductive structure)

• no parapodia, setae reduced/lost 

• all monoecious 

  • only contribute either an egg or sperm 

• all have direct development 

  • no trochophore larva 

Clitellata “Oliochaetes”

• earthworms  

  • freshwater, marine, and terrestrial

• well developed circulatory, digestive, and excretory systems

  • nepridia (primitive kidneys)

• ecologically important

  • bioturbation, soil/nutrient cycling, oxygenation

Simultaneous hermaphrodite 

• oliochaetes are simultaneous hermaphrodites, meaning they can exchange sperm and eggs together at the same time

Hirudinea

• annelida → sedentaria → clitellata → hirudinea  

• leeches 

• many freshwater, some marine, some terrestrial 

• dorso-ventrally flattened 

• fixed # of segments

• clitella only during reproduction

  • lost/reduced setae/chaetae

  • lost/reduced septa

• posterior and anterior suckers

  • powerful pharynx

  • blood sucking or carnivorous 

Sipuncula

• seperate from all other errantia, sedemtaria annelida, these worms are not segmented and have no setae

• peanut worms 

• trochophore larve unites them with the greater lophotrochozoans 

Annelid organization

• organ systems 

Annelid symmetry

• bilateral 

Annelid body cavity

• eucelomate 

Annelid development 

• triploblasts 

Annelid segmentation

• true segmentation