Chp 8

• Annelids


  Ringed worms or segmented worms, a large phylum with over 17,000 species including ragworms, earthworms, and leeches in marine, freshwater and terrestrial systems

  
• Annelids


  • Coelomates
  • Unquestionably segmented with repeated segments visible as rings, segments separated by partitions called septa, each segment has its own coelom
  • Fluid inside coelom of each acts as a hydrostatic skeleton, muscles push against the fluid, expanding and contracting (each segment acts independently)
  • Anterior segments often modified as heads, with sense organs, a brain, and even some with eyes
  • Some segments modified as copulatory organs
  • Connections between segments: nervous system via ventral nerve cord, circulatory system blood vessels (closed circulatory system)
  • Digestive tract with separate mouth and anus
  • Most have setae - bristles of chitin used to anchor the worm to a substrate ("bristleworms")
  • Closed circulatory system
• Classes of Annelida


  • Polychaeta (polychaetes)
  • Oligochaeta (earthworms)
  • Hirudinea (leeches)
• Clitellata


  Superclass that includes Oligochaeta and Hirudinea, have a unique reproductive organ, the ring-shaped clitellum ("pack saddle") around their bodies, which produces a cocoon that stores and nourishes fertilized eggs until they hatch
• Class Polychaeta


  • Around 10,000 species, mostly marine, often live in burrows
  • Have a well developed head bearing simple eyes, tentacles, palps or radioles
  • With parapodia - paired, fleshy, paddle-like flaps on most segments- used for moving and gas exchange
  • Sexes separate, but usually lack permanent gonads (sex organs); instead, make gametes from cells lining coelom or septa
  • Dioecious with external fertilization
  • Trochophore larvae - similar to that of mollusks (ciliated)

  
• Notable polychaetes


  • Pompeii worm (Alvinella pompejana) - endemic to hydrothermal vents, among the most heat-tolerant complex animals known

  
• Class Oligochaeta (earthworms)


  • Mostly terrestrial (in moist soil); a few aquatic species
  • Eat their way through soil, feed on decomposing vegetation and microscopic micro-organisms
  • 100-175 segments, with mouth on first one and anus on last one, with few setae, no appendages
  • Head is reduced with no eyes, but some light-sensitive organs near end of body
  • Hermaphroditic: join in opposite directions at the clitellum, exchange sperm, then each lays eggs in mucous cocoon
• Oligochaeta


  • Lumbricus terrestris (earthworm)

  
• Class Hirudinea (leeches)


  • Mostly freshwater species; some marine, some terrestrial
  • Include parasites, predators, and scavengers
  • Have enlarged terminal suckers for locomotion and attachment
  • Travel by looping movements of the body using the suckers for attachment, or they move like oligochaetes, and some swim by undulation of the body
  • Hermaphroditic with clitellum (only during breeding season)
  • Largely nocturnal, but may be attracted by food during the day
  • Lack setae: hence sometimes this class is called achaeta
  • Reduced segmentation (34 segments), presence of suckers, loss of seta, the reduction of coelom and the presence of copulatory organs and the genital openings on the midventral line of the body
  • Usually dorsoventrally flattened (resemble flatworms)
  • Many suck blood (external blood-sucking parasites), with chitinous jaws used to rasp through skin
• Hirudin


  Salivary enzyme that prevents coagulation of blood in leeches
• Digestion in leeches


  1. Liquid is soon excreted, but the concentrated part of such a 'food meal' remains fluid and is absorbed slowly over a period of several months

  2. Digestion is by proteolytic enzymes
• Leech anatomy


  • Body covered by cuticle secreted by epidermis, with many unicellular mucus glands
  • Muscular system with circular, longitudinal, oblique, and dorso-ventral bands of fibers
  • Mesenchyme between muscles and internal organs reduces coelom to a system of canals and sinuses
  • Digestive tract includes mouth, muscular pharynx with salivary glands and jaws, esophagus, long crop with lateral pouches, slender intestine, short rectum, and anus
  • Circulatory system composed of longitudinal sinuses and vessels, dorsal and ventral sinuses, and two lateral vessels, with many cross-connections
  • Respiration via network of capillaries beneath epidermis
  • Excretion by up to 17 pairs of nephridia, with nephrostomes opening into non-ciliated capsules that produce coelomocytes
  • Nervous system with dorsal ganglia, paired connectives to ventral nerve cord with segmental ganglia, and sensory structures including taste cells, tactile organs, eyes, and other sensillae
  • Male reproductive system with 9 pairs of testes, ductus deferens, seminal vesicle, ejaculatory duct, and penis
  • Female reproductive system with two ovaries, oviducts, albumen gland, and median vagina

  
• Leeches do not reproduce asexually and do not regenerate lost parts
• Annelida


  Ringed worms or segmented worms

  
• Annelida is a large phylum, with over 17,000 species including ragworms, earthworms, and leeches in marine, freshwater and terrestrial systems
• Annelida


  • Coelomates
  • Unquestionably segmented
  • Repeated segments visible as rings
  • Segments separated by partitions called septa
  • Each segment has its own coelom
  • Fluid inside coelom of each acts as a hydrostatic skeleton
  • Muscles push against the fluid, expanding and contracting (each segment acts independently)
  • Anterior segments often modified as heads, with sense organs, a brain, and even some with eyes
  • Some segments modified as copulatory organs
• Connections between segments: nervous system via ventral nerve cord, circulatory system blood vessels (closed circulatory system)
• Annelida have a digestive tract with separate mouth and anus
• Most Annelida have setae – bristles of chitin used to anchor the worm to a substrate ("bristleworms")

  
• Annelida have a closed circulatory system with some enlarged vessels that serve as hearts
• Annelida exchange gases at skin (no gills or lungs)
• Annelida have an excretory system that includes nephridia (like in mollusks): 2 per segment that transport waste out of coelom by excretory tubules
• Classes of Annelida


  • Polychaeta (polychaetes)
  • Oligochaeta (earthworms)
  • Hirudinea (leeches)
• Oligochaeta and Hirudinea are sometimes grouped in the superclass Clitellata
• Clitellata have a unique reproductive organ, the ring-shaped clitellum ("pack saddle") around their bodies, which produces a cocoon that stores and nourishes fertilized eggs until they hatch
• Class Polychaeta


  • Around 10,000 species, mostly marine, often live in burrows
  • Can have unusual and colorful forms; include plumed worms, peacock worms, fan worms

  
• Polychaeta characteristics


  • With a well developed head bearing simple eyes, tentacles, palps or radioles
  • With parapodia - paired, fleshy, paddle-like flaps on most segments- used for moving and gas exchange
  • Sexes separate, but usually lack permanent gonads (sex organs); instead, make gametes from cells lining coelom or septa
  • Dioecious with external fertilization
  • Trochophore larvae – similar to that of mollusks (ciliated)
• Polychaete ecology


  • Swim among the plankton or above the abyssal plain
  • Burrow or build tubes in the sediment
  • Live as commensals
  • A few are parasitic
  • Mobile forms (subclass: Errantia) tend to have well-developed sense organs and jaws
  • Stationary forms (subclass: Sedentaria) lack them, but may have specialized gills or tentacles used for respiration and deposit or filter feeding, e.g., fanworms

  
• Notable polychaetes


  • Pompeii worm (Alvinella pompejana) - endemic to the hydrothermal vents of the Pacific Ocean, among the most heat-tolerant complex animals known

  
• Class Oligochaeta (earthworms)


  • Mostly terrestrial (in moist soil); a few aquatic species
  • Eat their way through soil (usually eat own weight in soil each day; most soil has passed many times through worm guts)
  • Feed on decomposing vegetation and microscopic micro-organisms
  • 100-175 segments, with mouth on first one and anus on last one
  • With few setae. No appendages
  • The head is reduced with no eyes, but some light-sensitive organs near end of body
  • Hermaphroditic: join in opposite directions at the clitellum (obvious thickened band), which secretes mucus that holds the pair together during copulation. They exchange sperm, and then each lays eggs in mucous cocoon surrounded by chitin that is secreted by the clitellum (this cocoon protects the fertilized eggs)

  
• Common example of Oligochaeta


  • Lumbricus terrestris (earthworm)

  
• Class Hirudinea (leeches)


  • Mostly freshwater species; some marine, some terrestrial
  • Includes parasites, predators, and scavengers
  • They have enlarged terminal suckers for locomotion and attachment
  • They travel by looping movements of the body using the suckers for attachment, or they move like oligochaetes, and some swim by undulation of the body
  • Hermaphroditic with clitellum (only during breeding season)
  • They are largely nocturnal, but may be attracted by the food during the day
  • Their annelid characteristic includes a ventral nerve cord with segmental ganglia and the segmentally arranged nephridia, and gonads in the reduced coelom
  • They differ from other annelids in the lack of setae: hence sometimes this class is called achaeta. They also differ by the reduced segmentation (34 segments), presence of suckers, loss of seta, the reduction of coelom and the presence of copulatory organs and the genital openings on the midventral line of the body
  • Usually dorsoventrally flattened (resemble flatworms)
  • Many suck blood (external blood-sucking parasites), with chitinous jaws used to rasp through skin
• Hirudinea form a group with Oligochaeta that is sometimes called superclass Clitellata
• Structure of Hirudo medicinalis (leech)


  • Body at rest is long or oval in outline and usually flattened dorsoventrally; it is very flexible and may be stretched, contracted or dilated
  • Consists of 34 somites, and the exterior is marked by transverse furrows into many annuli (1-5 per somite)
  • At the posterior end (formed from seven somites) is a rounded sucker, and there is another anterior sucker around the mouth that is smaller than the posterior one
• Body wall of Hirudo medicinalis


  • Covered by a cuticle secreted by a single-layered epidermis, and many unicellular mucus glands open on the surface
  • Beneath is a dermis with pigment cells and blood capillaries
  • Muscular system is elaborated with circular, longitudinal, oblique, and dorso-ventral bands of fibers
  • Mesenchyme between the muscles and internal organs reduces the coelom to a system of canals and sinuses, from which the nephridia drain and in which the gonads and major blood vessels are located
• Digestive system of Hirudo medicinalis


  • Includes: mouth, muscular pharynx with unicellular salivary glands, and three horny toothed jaws, short esophagus, long crop with up to 20 pairs of lateral pouches or caeca, slender intestine, short rectum, and anus opening dorsally before the posterior sucker
  • Leeches are scavenging, predatory, or parasitic; some feed on dead animals and others prey on snail worms, insects, larvae and mollusks
  • Most are fluid-feeding animals, and are best known for the blood-sucking habits
  • A salivary enzyme (hirudin) prevents coagulation of blood
  • Much of the liquid is soon excreted, but the concentrated part of such a 'food meal' remains fluid and is absorbed slowly over a period of several months
  • Digestion is by proteolytic enzymes
  • Because leeches consume large amount of food, and since digestion is slow, a period of a year can pass between feedings
• Circulatory and respiratory systems of Hirudo medicinalis


  • Circulatory system is composed of longitudinal sinuses and vessels, dorsal and ventral sinuses, and two lateral vessels, with many cross-connections
  • Respiration is by a network of capillaries beneath the epidermis
• Excretory system of Hirudo medicinalis


  • Excretion is by up to 17 pairs of nephridia that sometimes are branched and sometimes have closed nephrostomes
  • As a result of the coelom reduction and loss of septa in the leech body, the nephridial tubules are embedded in connective tissue
  • Each nephrostome opens, usually, into a non-ciliated capsule. The function of the latter is the production of coelomocytes, which are phagocytic and engulf particulate matter
  • The nephridia open to the exterior through 17 pairs of nephridiopores (segments 7-23)
• Nervous system of Hirudo medicinalis


  • Resembles that of other annelids, with a pair of dorsal ganglia and the paired connectives to the ventral nerve cord, which has segmental ganglia
  • Four of the anterior ganglia and seven at the posterior end are fused
  • Each ganglion gives off several pairs of nerves
  • Sensory structures include taste cells in the mouth, tactile organs, 1-5 pairs of eyes anteriorly, and other eyelike organs (sensillae) on several annuli
• Reproductive system of Hirudo medicinalis


  • Male reproductive system includes 9 pairs of testes (segments 12-21) beneath the crop, those on each side join a ductus deferens running anteriorly and leads into a seminal vesicle, followed by the ejaculatory duct, and the two ducts enter a median penis to which the prostate gland connect
  • Female reproductive system comprises two ovaries and two oviducts joining a single albumen gland and a median vagina that opens just behind the male pore (segment 12)