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A First Look at Polychaetes

By: Gail Carmack
AEGCarmack@aol.com or carmack@tenet.edu

Purpose

To observe the body plan of representative polychaetes and to
describe their life style based on their structure.

Introduction

Polychaetes are a diverse class of annelids. Most polychaetes are 1-5 cm long, but some may be as long as 3 meters. There are over 5000 species of polychaetes and almost all are marine animals. Polychaetes may be pelagic, surface dwellers, burrowers, or tube dwellers. They may also be iridescent, luminescent, or brightly colored.

In the generalized polychaete body plan, only the head and tail are specialized. The rest of the segments are identical. The head, called the prostomium, consists of the first two segments, 2 pairs of eyes, 3 antennae, several tentacles, and a pair of palps. The mouth is located on the ventral side between the prostomium and the first trunk segment. The last segment, which is where the anus is located, is the tail, or pygidium. Growth in polychaetes occurs by adding segments just ahead of the pygidium. Each segment between the head (prostomium) and the tail (pygidium) bears two fleshy appendages called parapodia. Chitinous bristles called setae are embedded in the parapodia. Setae come in many shapes and are used to help the worm move by holding non-moving parts of the worm in place. Some polychaetes (e.g., the fireworm) excrete poison on their setae and use them for defense.

Few polychaetes exhibit the generalized body plan described above. Most have modified body plans to best meet the needs of their lifestyles. Mobile polychaetes come closest to the "general" body form. Sessile polychaetes often lack parapodia and have reduced head and sensory organs. Sessile polychaetes may also have bodies that have distinct regions (i.e.., a head, a trunk, and a tail).

Materials needed

  • Several preserved or live specimens of polychaetes. I use Nereis, Arenicola, Chaetopterus, Sea Mouse, and Beak Thrower.
  • Dissecting scopes
  • Probes
  • Culture dishes

Procedure

A. Preserved specimen #1
  1. Carefully remove the polychaete from the jar. Observe the overall body plan.

  2. Carefully examine your specimen and draw a dorsal view of the head and first few segments. Identify the palps, antennae, eyes, parapodia, and prostomium. (Note: the specimens may not have the exact same numbers of antennae and tentacles as in diagrams.)

  3. Locate the mouth on, and draw the ventral view. Label the mouth, antennae, tentacles, and palps.

  4. Look at the specimen under the dissecting microscope. Draw the setae.

  5. Is this polychaete mobile or sessile? Give evidence to support your conclusion.

  6. Is this polychaete a predator or a filter feeder? Give evidence to support your conclusion.


B. Examine Specimens #2-5.
  1. Examine each specimen by repeating the instructions for specimen #1. Answer the following questions for each specimen.

  2. Are the setae the same shape as specimen #1.

  3. Is this polychaete mobile or sessile? Give evidence to support your conclusion.

  4. Is this polychaete a predator or a filter feeder? Give evidence to support your conclusion.

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