Annelids and Allied Taxa

1. Annelids and Allied Taxa

3. Dividing up the Body • The fluid-filled coelom is an efficient hydrostatic skeleton • When septa divided this coelom into a series of compartments, metamerism first arose • Evolutionarily, if one segment should fail, another could still function • Metamerism makes independent segmental movements possible

4. Characteristics • The three phyla are Annelida (marine and freshwater worms, leeches), Echiura, and Spincula • All three phyla have a trochophore larva • We will spent most of our time understanding the phylum Annelida

5. Annelids • About 15,000 species of segmented worms • 2/3 of them are obscure, marine species not noticed very easily • The nervous system is more centralized and the circulatory system is fairly complicated • Except for leeches (Class Hirudinea), annelids have tiny chitinous bristles called setae • Short setae anchor a segment in an earthworm (for example) to prevent it from slipping backward • Long setae help aquatic worms swim

6. The ecology of annelids • They occur in the sea, freshwater, and on land • Marine annelids borrow in the mud or live in tubes • Some feed on organic matter in the mud, some are suspension feeders, many are predators • Freshwater annelids burrow in the mud, live among vegetation, or swim freely • Leeches are bloodsuckers or are carnivorous

7. Economic importance • Many annelids are grazers and consumers of detritus in food chains; many are preyed upon by fish • Burrowing on land or in oceanic mud and sand, annelids are important for drainage, aeration, mixing of soil and distribution of organic matter • A study by Charles Darwin showed that the lowly earthworm brings as much as 8 -20 tons of soil to the surface each year • Blood-sucking leeches are used medicinally

8. Anatomical Features • Two part head has anterior tip containing the prostomiumand peristomium • Terminal portion bearing the anus is the pygidium • The prostomium and pygidium are not true segments • New metameres form just in front of the pygidium; thus the newest segments are at the posterior

9. Anatomical Features • Peritoneum (mesodermal epithelium) lines the body wall and forms dorsal and ventral mesenteries • Septa are between adjacent segments • Except in leeches, the coelom is filled with fluid and serves as a hydrostatic skeleton • The fluid volume remains constant • Therefore contraction of longitudinal muscles causes the body to shorten and fatten

10. How annelids move • Contraction of circular muscles causes the body to narrow and lengthen • By separating this force into sections, widening and elongation move the whole animal • Alternate waves of contraction or peristalsis result in efficient burrowing • Swimming annelids use undulatory movements

11. Anatomy of the Annelids

12. More Anatomy

13. Class Polychaeta (the bristle worms) • The largest class of annelids with more than 10,000 species, mostly marine • From 1 mm to 10 feet long • Some live in crevices or in tubes; some swim and are pelagic • They are an important component of the food web • Polychaetes have a well differentiated head with sensory organs • Paired appendages called parapodia are on most segments

14. Characteristics of the polychaetes • They have no clitellum as in earthworms • Many setae are arranged in bundles on the parapodia • The head or prostomium may or may not be retractable; it often bears eyes, tentacles and sensory palps • The first segment or peristomium surrounds the mouth and may have setae, palps, or chitinous jaws • Suspension feeders may bear a tentacular crown that opens like a fan but can be withdrawn into a tube

15. Segments of the trunk • Most segments bear parapodia with lobes, cirri, setae and other parts • Parapodia have two lobes, the dorsal notopodium and the ventral neuropodium • Usually the parapodia are the chief respiratory organ although the worm may also possess gills • The parapodia help the worm to crawl, swim or serve as an anchor in a tube • Along with gills, they serve as chief respiratory organs • Amphritite has plumelike gills

16. Amphitrite • Builds its tubes in mud or sand • Extends long tentacles over the mud to gather bits of organic matter as food

17. Tube-dwelling sedentary polychaetes • The Christmas tree worm, Spirobranchusgiganteus, lives is a calcareous tube • The sabellidpolychaete, Bispirabrunnea, lives in a leathery tube

18. Sense organs of polychaetes • Sense organs include eyes and statocystes that are more developed than in the class Oligochaeta • The eyes vary from simple eyespots to well developed eyes that can distinguish images

19. Reproductive characteristics • Polychaetes have no permanent sex organs and sexes are separate • The gonads appear as temporary swellings of the peritoneum • Gametes are shed into the coelom and exit by gonoducts, metanephridia or rupturing of the body • Fertilization is external and the early larva is a trochophore

20. Diversity of the Polychaetes • These worms are pelagic, burrowers, sometimes sedentary some live in tubes • Nereis, the clam worm, is a predatory polychaete with jaws which can deliver a noticeable bite when handled • Featherduster worms or fan worms live in tubes but unfurl tentacular crowns to feed • Fanworms are polychaeteciliary feeders, directing small food balls along grooved radiolesto the mouth by ciliary action

21. Sabella, a polychaete suspension feeder

22. Chaetopterus • A sedentary polychaete which has wing-like notopodia that secrete mucous filters to strain out food particles from the water • As the net fills with food, the food cup rolls it into a ball which is passed into a groove toward the mouth

23. Arenicola • This polychaete lives in an L-shaped burow in intertidal mudflats and ingests food-laden sand

24. Reproductive behavior of polychaets • Most of the time, polychaetes live as sexually immature worms called atokes • When swollen with gametes, they are called epitokes • At the beginning of the last quarter of the October-November moon, the polychaetes enter a “swarming period” • The epitokes break off and swim to the surface; swarming ensures that all epitokes mature at the same time

25. Class Oligocheta • Over 3000 species occur in habitats from soil to freshwater; a few are marine or parasitic • Nearly all have setae but in general, less setae than polychaetes • Earthworms burrow in moist, rich soil, emerging at night • In wet weather, they stay near the surface. When it is dry, they burrow deep and become inactive

26. More on earthworms • Lumbricusterrestrisis often studied in biology classes • Charles Darwin studied earthworms and estimated that 10 to 18 tons of dry earth passed through earthworm intestines per acre annually • The worms help churn the soil, mixing materials, and adding nutrients • Giant tropical earthworms may reach 4 meters long and live in interconnected tunnels

27. Form and function of oligochaetes • The prostomium overhangs the mouth at the anterior end • In most earthworms, each segment bears four pairs of chitinous setae, some may bear over 100 • Each seta is a bristle-like rod set in a sac and moved by tiny muscles • The setae anchor segments during burrowing

28. How earthworms move • Earthworms move by peristalsis • Circular muscles contract, the anterior end lengthens, setae anchor the forward end • Longitudinal muscles contract, body shortens, and the posterior end is pulled forward

29. Feeding habits of Oligochaetes • Most are scavengers, feeding on decayed organic matter, leaves, refuse, etc • Food is moistened by the mouth and drawn in by a sucking action of the muscular pharynx • Digestion and absorption occur in the intestine; an infolded typhlosole increases the surface area • Chloragogen tissue surrounds the intestine and helps in digestion • The chloragogen cells also function in excretion

30. Circulation, respiration, and excretion • Both coelomic fluid and blood carry food, wastes and respiratory gases • Blood circulates in a closed system with five main trunks running lengthwise in the body • The dorsal blood vessel above the alimentary canal has valves and functions as a true heart • The dorsal vessel pumps blood anteriorly into five pairs of aortic arches • Blood contains colorless amoeboid cells and dissolved hemoglobin

31. Circulation, respiration, and excretion • Gaseous exchange (oxygen and carbon dioxide) occurs through the moist skin • Each somite (segment) except the first three and the last one have a pair of nephridia • Each nephridium occupies parts of two adjacent segments • A ciliated funnel, the nephrostome, draws in wastes and leads through the septum • These coil until the nephridial duct ends at a bladder that empties outside at the nephridiopore • Wastes from the coelom are discharged

32. Nervous system and sensory organs • A pair of cerebral ganglia connects around the pharynx to the ganglia of the ventral nerve cord • Fused ganglia in each somite contain both sensory and motor fibers • One or more giant axons are located in the ventral nerve cord to increase the rate of conduction and stimulate contractions of muscles in many segments

33. Reproduction and Development