Echinoderms Study Guide

Annelids, Nematodes, Platyhelminthes, and Mollusks.

 Invertebrate Review Sheet

Platyhelminthes (Flatworms)

Examples: planaria, tapeworms, flukes

Key Features:

• Flattened body

• No body cavity (acoelomate)

• Bilateral symmetry

• Incomplete digestive system (one opening) 
  tapeworms diffuse food particles through their body without a structural digestion system

Other Traits:

• Simple nervous system (ladder-like)

• Many are hermaphrodites

• No circulatory or respiratory systems

Importance:

• Many are parasites (tapeworms, liver flukes)

• Planaria cut in half can regenerate into two fully functional organisms 

Nematodes (Roundworms)

Examples: Ascaris, hookworms, pinworms

Key Features:

• Unsegmented, cylindrical body

• Pseudocoelom (partial body cavity)

• Bilateral symmetry

• Complete digestive system 

• No circulatory or respiratory system

Other Traits:

• Tough outer covering (cuticle)

• Molt (shed cuticle)

Importance:

• Many are parasitic (infect humans, animals, plants)

• Some are free-living in soil/aquatic environments

Annelids (Segmented Worms)

Examples: earthworms, leeches, polychaetes (marine bloodworms)

Key Features:

• Segmented body (repeating units)

• True coelom (body cavity fully lined with tissue)

• Bilateral symmetry

• Closed circulatory system 

• Complete digestive tract, one way system (mouth → anus)

Other Traits:

• Setae (tiny bristles) for movement

• Well-developed organs (brain, nephridia for excretion)

Importance:

• Earthworms improve soil fertility

• Some are parasites (leeches)

Mollusks

Examples: snails, clams, squids, octopuses

Key Features:

• Soft body, often with shell

• True coelom (reduced)

• Bilateral symmetry

• Complete digestive system 

Body Plan (3 parts):

1. Foot – movement

2. Visceral mass – organs

3. Mantle – may secrete shell

Other Traits:

• Most have open circulatory system 
  (exception: cephalopods like squid have closed systems)

• Radula (scraping feeding structure, not in all)

Importance:

• Major food source (clams, oysters)

• Some highly intelligent (octopuses)

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Summary Comparison Chart