Annelids

• Phylum Annelida

  Segmented Worms
• Annelids


  • Multicellular
  • With tissues
  • 3 germ layers: bilateral symmetry
  • Protostome development
  • Coelom
• Annelids


  • Protostomic
  • Triploblastic
  • Schizocoelous
  • Coelomate - first group to develop a true coelom (body cavity)

  
• Schizocoelous formation

  Splitting of the mesoderm to form two coelomic cavities, one on either side of the body
• Coelom


  A fluid filled space within the mesoderm
• Importance of coelom development

  • Associated with an increase in animal size - large body and fluid volumes are typical
• Vermiform body


  Worm shaped, soft bodied, circular in cross section, longer than they are wide
• Epidermis and Dermis

  • Epidermis - one-cell deep outermost skin layer, secretes cuticle
  • Dermis - under the epidermis, made of connective tissue
• Muscles

  • Two layers - circular muscles and longitudinal muscles, derived from the lining of the coelom
• Setae


  Small, non-articulated bristles that provide traction and stability
• Chaetae


  Larger, articulated bristles used for defense, locomotion, or sensory purposes
• Setae vs Chaetae

  • Setae are more primitive and found in a wider range of annelids, chaetae are more specialized and found mainly in polychaetes
• Beta-chitin


  One of the two main crystalline forms of chitin, found in the cuticles of some annelid species
• Parapodia


  • Unjointed paired extensions of the body wall that function as limbs, found in nearly all polychaetes
• Metameric Segmentation


  • Body consists of serially repeating, coordinated segments, each with integumentary, muscular, nervous, circulatory, reproductive, and excretory organs
  • Allows greater flexibility of movement
• Prostomium


  Frontmost section containing the brain and sense organs
• Pygidium or Periproct


  Rearmost section containing the anus
• Peristomium


  First section behind the prostomium, bears the mouth
• Teloblastic growth


  Segments develop one at a time from a growth zone just ahead of the pygidium, with the youngest segment just in front of the growth zone and the oldest at the anterior end
• Segmentation in different annelid groups

  • Leeches have a fixed maximum number of segments
  • Non-leech clitellates add many more segments after hatching
  • Non-clitellate annelids have variable larval segments and add many more in post-larval development
• Septa


  Thin sheets of mesodermally derived tissues that isolate coelomic fluid in one segment from adjacent segments
• Annelids (segmented mollusks)


  • Segments separated by septa
  • Thin sheets of mesodermally derived tissues (peritoneum) isolate coelomic fluid in one segment from that in adjacent segments
  • Allow for localized deformation
• Locomotion in polychaetes

  1. Parapods (fleshy lobes supported by chitinous rods) on each side of the animal

  2. Parapods have one or more pairs of chitinous setae or chaetae which can be sharp (for protection) and aid in locomotion
• Locomotion methods

  • Peristalsis (waves of contraction and expansion that sweep along the body) - earthworms
  • Flexing the body while using parapodia to crawl or swim - polychaetes
  • Looping movement - leeches
• Respiration


  • Cutaneous - gas exchange through skin
  • Branchial - through gills associated with most segments (extensions of the parapodia)
  • Gills of tube-dwellers and burrowers usually cluster around whichever end has the stronger water flow
  • Respiratory pigments: Hemoglobin, Chlorocruorin, Hemerythrin
• Circulation


  • Blood flows entirely in closed vessels
  • Some species have several hearts
  • Blood contains hemoglobin, which increases oxygen carrying ability
  • Closed circulatory system - with blood vessels from the heart to different parts of the body, then back to the heart
• Pseudohearts


  Contractile vessels that help circulate blood and oxygen throughout the body, found in earthworms
• Hearts


  True hearts that pump blood throughout the body, found in leeches
• Feeding structures

  • Muscular pharynx that can be everted - in many polychaetes
  • Jaws - in 2 families, Eunicidae and Phyllodocidae, can be used for seizing prey, biting off pieces of vegetation, or grasping dead and decaying matter
  • Neither jaws nor eversible pharynges - in some predatory polychaetes
  • Palps - in selective deposit feeders that generally live in tubes on the sea-floor, to find food particles in the sediment and then wipe them into their mouths
  • "Crowns" of palps covered in cilia - in filter feeders, to wash food particles towards their mouths
  • Mouths that are generally unspecialized - in non-selective deposit feeders, to ingest soil or marine sediments
  • Sticky pads in the roofs of their mouths - in some clitellates, some can evert the pads to capture prey
  • Eversible proboscis or a muscular pharynx with two or three teeth - in leeches
• Gut


  • Almost straight tube supported by mesenteries (vertical partitions within segments)
  • Ends with the anus on the underside of the pygidium
  • In tube-dwelling family Siboglinidae, gut is blocked by a swollen lining housing symbiotic bacteria that convert inorganic matter to organic matter that feeds the worms
• Excretion

  • Some wastes excreted across the body surface
  • Has 2 nephridia (metanephridium) that open at both ends to discharge excretory wastes and gametes
  • Coelomic fluid is drawn into the nephridium at the nephrostome, substances may be selectively resorbed or actively secreted, regulates water content of coelomic fluid
  • Ducts from gonadal tissue merge with nephridial tubule, so nephridia play a role in discharging gametes as well as urine
• Nervous system


  • Brain forms a ring around the pharynx, consists of a pair of ganglia above and in front of the pharynx linked by nerve cords to another pair of ganglia below and behind it
  • "Ladder-like" central nervous system with a solid nerve cord (or pair of cords) running through the body with segmental ganglia
  • Each muscle fiber is controlled by more than one neuron, longitudinal nerve trunks include giant axons for rapid withdrawal from danger
  • Sense organs include nuchal organs, ocelli, statocysts, palps, and antennae
• Asexual reproduction methods

  • Binary or multiple fission - polychaetes, oligochaetes
  • Budding - polychaetes, e.g. sabellid tubeworm
  • Regeneration - for reproduction or repair after damage - polychaetes, oligochaetes
• Sexual reproduction

  • Dioecious or monoecious, some can change sex during life
  • Sexes usually separate with gonads in each segment, some have gonad specific segments
  • Fertilization can be internal or external
  • Sperm transfer methods: females collect sperm released into water, males have penis that injects sperm into female, some store sperm in spermathecae
• Earthworm copulation and egg cocoon formation

  Mutual insemination, clitellum secretes mucous tube and tough band that forms cocoon, cocoon passes forward to receive eggs and sperm, cocoon deposited near burrow entrance, young worms emerge in 2-3 weeks
• Reproduction in polychaetes

  • Females collect sperm released into water, males have penis that injects sperm into female
  • Some breed only once in their lives, others almost continuously or through several breeding seasons
  • Most are gonochoristic all their lives, some are full hermaphrodites or change sex
  • Most lack permanent gonads, uncertain how they produce ova and sperm
  • Epitoky - rear of body splits off and becomes a separate individual that lives long enough to swim to a suitable environment
• Developing cocoon

  1. Passes forward to receive eggs from oviducts and sperm from seminal receptacles

  2. As cocoon slips off over anterior end, its ends close and seal

  3. Cocoon is deposited near burrow entrance

  4. Young worms emerge in 2 to 3 weeks
• Two earthworms in copulation. Their anterior ends point in opposite directions as their ventral surfaces are held together by mucous bands secreted by the clitella.

  
• Reproduction in polychaetes

  • Females collect sperm released into the water; males have a penis that inject sperm into the female
  • Some only breed once in their lives; others almost continuously or through several breeding seasons
  • Most are gonochoristic all their lives; some are full hermaphrodites, or change sex during their lives
  • Most lack permanent gonads—uncertain how they produce ova and sperm
  • Epitoky in a few species – rear of the body splits off, becomes a separate individual that lives long enough to swim to a suitable environment, usually near the surface, and spawn
• Reproduction in clitellates

  • Most mature clitellates are full hermaphrodites, although in a few leech species younger adults function as males and become female at maturity
  • Have well-developed gonads, and all copulate
  • Cocoon also either produces yolk when the eggs are fertilized or nutrients while they are developing
  • Hatch as miniature adults rather than larvae