Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Biology 320 Invertebrate Zoology Fall 2005 Chapter 13 – Phylum Annelida Part One Phylum Annelida Name means “ringed” Mostly marine, but also freshwater and terrestrial representatives Size varies greatly Microscopic – 3 m (giant Australian earthworm) 12,000 spp. Segmented Diverse modes of feeding Body Regions Three main regions (anterior to posterior) Prostomium Trunk Anus Some sensory appendages Have annulations (superficial segments), but also true segments Majority of body segments Pygidium Head with sensory organs Peristomium with mouth Internal compartmentalization Growth zone is located just anterior to pygidium Segments added posteriorly Oldest segment is most anterior Some organs / structures are found in every segment Other organs are parts of integrated systems Appendages Coelomic cavities Nephridia Gonads Digestive system Nervous system Hemal system Musculature Segments are separated transversely by septa Bilateral coelomic cavities are also partitioned longitudinally by mesenteries Generalized Body Wall Cuticle Protective layer that resists abrasion and helps prevent desiccation Annelids are still restricted to moist environments Epidermis Glandular for production of mucus Previously discussed functions of mucus apply here Produce chaetae Chitinous bristles that project from epidermis Used for traction Not to be confused with setae of arthropods, which are sensory hairs Connective tissue Musculature Circular and longitudinal Generalized Nervous System CNS Brain is located dorsally in prostomium Two ventral longitudinal nerve cords with giant axons Impulses travel up to 40X faster than neurons of average diameter Facilitate the escape response Segmental ganglia Commissures give the nervous system a ladder-like appearance Sensory structures Typical unicellular receptors distributed on head, body, and appendages Polychaetes posses eyes and nuchal organs (more later) Coelomic Cavities Two lateral coelomic cavities in each segment Transverse septa Two longitudinal mesenteries One ventral and one dorsal to gut Benefit is that musculature and hydrostat of individual segments can be precisely controlled Coelomic cavities may be lined with mesothelium containing chlorogogen cells Tissue appears yellow or brown in color Has important metabolic functions Synthesis and storage of glycogen and lipids Detoxification Hemoglobin synthesis Protein catabolism and formation of nitrogenous wastes Segmentation Advantages: Burrowing Isolation of hydrostat and muscular contractions Studies have compared fluid pressures in segmented worms vs. non-segmented worms that are burrowing Pressure is transmitted throughout coelom in nonsegmented In order to prevent aneurisms, all muscles must contract in order to antagonize pressure Generalized Hemal System Dorsal and ventral blood vessel, which are connected by capillary beds Blood is pumped by: Blood may contain some hemoglobin, but typically larger concentrations are contained in amoeboid coelomocytes Vessels Muscular contractions Enlarged portions of anterior dorsal blood vessel which function as hearts WBC + RBC hybrid Gas exchange occurs across body wall, appendages, and gills Generalized Excretory System Paired segmental nephridia Funnels or terminal cells project into coelomic fluid Highly coiled tubule Large surface area for secretion and absorption Nephridiopore opens to exterior of animal Generalized Reproduction Clonal Fragmentation Budding Paratomy Sexual Gonochoric External fertilization Gametes exit through nephridiopores Gonads are segmental, and housed in coelomic cavities Trochophore larvae, with growth zone just anterior to the telotroch Class Polychaeta Polychaetes or bristleworms Name means “many chaetae” 8000 spp. Marine, a few freshwater, and a few tropical terrestrial varieties Diverse feeding ecology, locomotion, and chaetae structure Also abundant (13,425 / m2 in Tampa Bay, FL) 2 mm to 3 m (Eunice) Body Form Posses appendages called parapodia Lateral outgrowths of body wall Supported by chitinous rod Each segment bears one pair Posses a chaetal sac which secretes a bundle of chaetae Chaetal structure is closely related to lifestyle and locomotion Errant (motile) Leg shaped – crawling Paddle shaped – swimming Shovel shaped – digging Sedentary Reduced – burrowing Hook shaped – also for burrowing Tubes Only annelids that secrete and occupy tubes Constructed from: Fibrous proteins Foreign materials Calcified by animal to form a shell Tubes may be attached to a surface or partially buried in sediment Several functions: Protection Hide-out from which prey can be ambushed Brooding chamber Sometimes tubes wash up on beaches Nervous System Typical of annelids Segmental pedal ganglia associated with parapodia Sensory organs Nuchal organs Ocelli Best developed in errant varieties, but useful for detecting shadows in sedentary worms Statocysts Unique to polychaetes Pair of eversible ciliated chemosensory pits on prostomium Useful to burrowers Sensory appendages Antennae – prostomium Palps – prostomium Cirri – peristomium, parapodia, and pygidium Digestive System Typical of annelids May have a protrusible pharynx With jaws for grasping prey May be used for burrowing Esophageal ceca secrete digestive enzymes Many strategies for waste removal, based on lifestyle Errant – leave wastes behind while moving Sedentary – interesting adaptations and behaviors to help avoid waste contamination Some live upside down in burrows Some invert themselves just while defecating (sensory appendages on pygidium are useful here) May create unidirectional water flow through the burrow tube Nutrition Once again, related to lifestyle Errant Burrowers Deposit feeders Sedentary (attached) Deposit feeders Tube dwellers Carnivores often have protrusible pharynx with jaws Herbivores or scavengers Suspension feed using feather crown with large SA Parasites Endoparasites of polychaetes and sea stars Ectoparasites that suck the blood of eels Gas Exchange Body wall Gills Exposed, delicate outgrowths of body surface Modified portions of parapodia Feeding crowns in sedentary polychaetes, such as feather-duster worms, double as gills Circulatory System Smaller varieties lack a hemal system, but possessed by larger animals Typical annelid system plus lateral parapodial vessels Variety of oxygen binding pigments can be found, especially in larger animals All function differently and have different oxygen affinities Intertidal polychaetes must cope with periods of emersion or stagnation of water Some undergo torpor – suppress metabolism Rely on stored oxygen from neuroglobin or myoglobin Excretion Typically have segmental nephridia Occasionally localized in one or a few segments Feather duster worms have one pair of anterior nephridia that empty via one medial nephridiopore, located on the head Tubes are coiled to increase SA for secretion / absorption Osmoregulate in low salinity (brackish) water by increasing rate of filtration Removes excess water A few species have even colonized freshwater Reproduction Clonal All forms of clonal reproduction Excellent regenerators Chaetopterus can grow an entire worm from one segment Sexual Most only reproduce sexually Most gonochoric Gonads are segmental in “abdomen” Gametes released into coelom Ripe gametes are shed via nephridiopores or rupture body wall Epitoky Reproductive phenomenon characteristic of many polychaetes Worms exhibit two life phases Atoke – benthic non-reproductive individual, which transforms into an… Epitoke – pelagic reproductive individual Epitokes arise from atokes via metamorphosis or budding Undergo several modifications and essentially become swimming sacs of gametes Enlargement of eyes, parapodia, and chaetae Become sexually mature, and gametes ripen Epitokes reach sexual maturity at the same time and swarm Swim to the surface and release gametes Many different cues: Female pheromone release Light cycles Lunar cycles Very predictable patters in some Some convert back into atokes after spawning; some reproduce once and die All regulated by hormones Oviposition Many shed eggs into sea Some attach gelatinous masses of eggs to substrata Chimney of tube Some brood eggs Retain eggs in tube or burrow Sac attached to ventral surface of parent Development Trochophore larvae in most Growth zone just anterior to telotroch May pass through trochophore phase in egg before hatching Some direct development Life spans Some live one or two years, and reproduce once (called annuals) Some live and breed for more than one year (perennial) Short life spans that progress through several generations in just one year (multiannual) Diversity Lugworms Construct L-shaped burrows Deposit feeders Arenicola Bamboo worms Construct tubes of sand that resemble drinking straws Deposit feeders Live upside down in tubes and ingest substrate from below Clymenella Paddle worms Paddle-like portions of parapodia are used as gills Errant – carnivores that hunt prey by crawling around Eteone tracks prey by following mucus trails Ragworms Errant – crawl, burrow, and swim Eversible, muscular pharynx with jaws Lots of cephalic sensory structures Some reach 1.8 m Nereis Bloodworms Live in shallow sedimentary burrows Ambush predators Capture prey with extremely long eversible pharynx Have poison glands Glycera Scaleworms Two rows of overlapping scales (resemble fish scales) on dorsal surface Scales are actually cirri Aphrodite (sea mouse) Covered with extremely long chaetae that resembles felt or hair Fireworms Brightly colored Resemble caterpillars Feed on corals and barnacles Calcified chaetae break off when touched Causes a painful, burning sensation Amphinome Shaggy tube worms Chimneys of tubes are ornamented with foreign materials collected by the worm Camouflage for tube Ambush predators Diopatra Parchment-tube worm Chaetopterus Lives in U-shaped burrow Filter feeder Seines plankton with mucus net it creates Net is rolled into a ball and swallowed Mucus is bioluminescent to ward of potential predators Spaghetti worms Occupy burrows Parapodia are reduced to grab sides of burrow Cluster of extensible tentacles Extend over sediment like active spaghetti Can be 1 m or more in length Mucus on tentacles traps particles Amphitrite Feather-duster worms Secrete calcareous tubes or construct from foreign materials Crown of tentacles (known as radioles) One radiole (most dorsal) is modified and acts as an operculum when crown is withdrawn Suspension feeders Beard worms Also called pogonophorans 80 spp. Live deep in ocean; typically near hydrothermal vents Up to 1.5 m in length Prostomium bears beard of tentacles Also live in tubes No mouth or anus Symbiotic chemoautotrophic (sulfur oxidizing) bacteria live in modified gut (trophosome)