BIO190: The Class: Segmented Worms: Lesson 18

The Segmented Worms

Phylum Annelida (segmented worms: three classes)

For additional information visit this site:
Worm World

General Characteristics of the phylum Annelida are:

• "little ring"

• metamerism

• secreted cuticle

• share characteristics with molluscs and arthropods

• eucoelomate

• cephalization
  1. anterior
     1. tentacles
     2. palps
     3. eyespots

• nervous system
  1. cerebral ganglia
  2. ventral nerve cords with lateral branches

• circulatory system
  1. closed
     1. "hearts" = aortic arches
     2. various respiratory pigments

• some with paired appendages
  1. respiratory
  2. locomotion

• excretory
  1. nephridia waste from
  1. blood
  2. coelom
  • digestion system
    1. complete
       (crop/gizzard present in some)

  • respiration
    1. cutaneous
    2. gills
    3. parapodia

  • reproduction
    1. sexual
       1. monoecious
       2. dioecious

  • trochophore larva (review from molluscs)

  • regeneration
    (capabilities in some)

  • segments
    (annuli)

  • setae
    1. "hair-like" projections (absent in leeches)
       1. locomotion
       2. reproduction
       3. swimming

    prostomium = terminal anterior

    pygidium = terminal posterior

    hydrostatic skeleton (except leeches)

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  Class Polychaeta

  The polychaetes comprise the largest class of annelids with more than 10,000 species, most of them marine.

  • " many hairs" = setae

  • largest class

  • mostly marine

  • euryhaline

  • well-defined anterior with sense organs (fig. 18.3)

  • paired appendages = (parapodia; see fig. 18.)
    1. most segments
    2. arranged in groups

  • no clitellum

  • nutrition

  • circulation system
    1. varies between species
       1. blood vessels
       2. respiratory pigments present

  • respiration system
    1. gills
    2. parapodia

  • excretion system
    (protonephridia or mix with metaneph.)

  • class is divided into 2 groups
    1. sedentary (figs. 18.4 & 18.5/18.10)
       (mostly in permanent burrows)

       (figs. 18-7 --> 18.9)
       

    2. free moving

  • nervous system
    1. dorsal ganglia
    2. ventral nerve cords

  • sensory
    1. eyes present in some
    2. nuchal organs
       (chemoreceptive pits)

    3. statocysts

  • reproduction
    1. dieocious
    2. fertilization ext.

  • development
    (trochophore)

  • LC Nereis (pg. 355-356)

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  Class Oligochaeta

  The more than 3000 species of oligochaetes are found in a great variety of sizes and habitats.

  • "few hairs"

  • mostly terrestrial or freshwater

  • setae present (fig. 18-12 & 18-13)

  • most common is earthworm (pg. 358-362)

  • nutrition
    1. scavengers
    2. calciferous gland

  • digestive system
    1. crop
    2. gizzard
    3. typhlosole (fig. 18-12)
    4. chloragogen tissue

  • circulation system (fig. 18-12 A)
    1. dorsal vessel

    2. ventral vessel
       (" aorta")

    3. 5 aortic arches
       (pressure in ventral vessel)

    4. respiratory pigment

  • respiratory system
    (cutaneous diffusion)

  • excretory system(fig. 18-14)
    1. most somite has metanephridia

    2. nephridiopore
       1. terrestrial (urea)
       2. aquatic (ammonia)

  • nervous system
    (central with peripheral nerves)

  • sensory
    1. sense organs
       1. distributed over body
       2. photoreceptors
          1. positive phototactic in low light
          2. negative phototactic in strong light

    2. reproduction (figs. 18-17 & 18-18)
       1. monoecious
          (x-fert.)

       2. clitellum
          (cocoon)

       3. development
          (direct; no metamorphosis)

    3. freshwater oligochaetes (pg. 361-362)

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  Class Hirudinea

  Leeches occur predominantly in freshwater habitats, but a few are marine.

  Leeches
  • fixed number of somites(fig. 18-21)
    (annuli)

  • nutrition
    1. mostly fluid feeders
    2. some parasitic
    3. some predaceous

  • circulation system
    1. some use oligochaete system
    2. some use coelomic sinuses

  • respiration system
    (cutaneous)

  • excretion system
    (nephridia)

  • nervous system
    1. two "brains"
       1. anterior
       2. posterior

    2. two nerve cords

  • sensory
    1. ocelli present
    2. sensillae
       (each somite)

  • reproduction
    1. monecious
       ( x-fert.)

  • significance of metamerism (pg. 365)

  • phylogeny/adaptive radiation (pg. 366)

  Once you have completed the lesson, you should go to Assignment 18-1.

  E-mail Sylvester Allred at Syl.Allred@nau.edu
  

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