Phylum Annelida Segmented Worms Phylum Annelida Metameric Body

Phylum Annelida Segmented Worms

Phylum Annelida • Metameric Body Plan – Bodies are divided into segments – Separated by septa – Each segment has own excretory, nervous and circulatory structures – Flexible support and efficient locomotion

Characteristics • • • Most complex of all worms Triploblastic, coelomates Bilateral symmetry More complex Closed circulatory system

Characteristics • Two sets of muscles – Circular and longitudinal • Paired epidermal setae (bristles) – Ventral side – Used in movement

Class Polychaeta “many bristles” ~5300 species Mostly marine Largest group Burrow into sea floor or live in reefs or tubes • Carnivores or filter feeders • Lateral extensions called parapodia with numerous setae • Respiratory gases diffuse across the body wall • • •

Reproduction • Remarkable regeneration – Some have break points that allow worms to sever themselves when a predator grabs them – Lost segments are later regenerated • Asexual reproduction – Budding or transverse fission • Sexual reproduction (most common) – Dioecious – Epitokes

Class Clitellata • Characterizes organisms based upon clitellum and few or no setae • Subclass Oligochaeta – Earthworms • Subclass Hirudinea – Leeches

Subclass Oligochaeta “few hairs” ~3000 species Freshwater/terrestrial habitats Lack parapodia and have fewer, shorter setae • Earthworms • •

Characteristics • Mostly live in terrestrial environments • Most have 100 -150 identical segments – Specialized segements 35 -37 (clitellum) • Reproduction • Moves by anchoring setae then contracting circular muscles

Digestion • Takes in soil through pharynx (acts as a pump) • Soil moves down esophagus to crop (storage) • Moves to gizzard (crushing/grinding) • Moves to intestine, absorption and digestion occurs through folds in walls • Solid waste out through anus • Valuable to gardeners and farmers – Break up soil, aerate, and add nutrients (Nitorgen)

Circulation • Closed circulatory system • Blood circulates through series of vessels • Blood absorbs molecules and carries them through dorsal vessel to five pairs of “hearts” • “Hearts” pump blood into main ventral blood vessel and smaller vessels carry blood to body.

Respiration and Excretion • Take in O 2 and give of CO 2 by diffusion through skin. • Must remain moist (not too wet or dry) • Eliminate liquid wastes through ciliated tubes called metanephridia (2 per segment) • Chloragogen tissue (acts like a liver) – Amino acid metabolism, converts ammonia to urea – Converts excess carbs into glycogen and fat (store energy)

Nervous Control • Can respond rapidly due to concentration of nerves called cerebral ganglion (anterior end). • Connected to rest of body by ventral nerve cord • Ganglion connects each segment to cord • No external eyes or ears • Receptors in skin will enable reactions (light, sound, chemical) – Active mainly at night • Giant Fibers – Mediate an escape response – Responds to stimulus from either end of worm – Initiates “anchor” and longitudinal muscles contract to quickly pull worm away from stimulus

Reproduction • Hermaphroditic • Female structures anterior, male structures posterior • Two worms exchange sperm – Mucus secretion from clitella holds them together – Sperm stored in seminal receptacle until just before eggs are laid – Clitellum forms a cocoon for the deposition of sperm and eggs – Fertilization occurs in the cocoon – Ends are sealed, deposited in soil, hatches in 1 -3 weeks

Subclass Hirudinea • ~500 species • Leeches • Predators (feed on small invertebrates or feed on body fluids) • 34 segments, reduced clitellum (only during reproduction) • Freshwater (some are marine or terrestrial) • Do not have setae

Characteristics • Flat and tapered body with suckers on each end • Attach with anterior suckers – Three sharp jaws to pierce host skin – Secrete “hirudin” that prevents hosts’ blood from clotting • Reproduce sexually (monoecious)

Interesting Facts • Used to be used to “detoxify” blood – Thought leeches removed toxins • Plastic surgery – Used to increase blood flow which keeps tissue alive long enough for surgery, re-attachment or tissue lifting