Biology 11Kingdom Animalia:Phylum Annelida:

The Segmented Worms

Coelomates• At this point in animal evolution, a true coelom

developed. So there is mesoderm - and therefore muscle - around the skin and the gut

• It also marks the point where two great lines of animal evolution diverged: the Protostomes and the Deuterostomes

The Protostomes• Contain three major phyla of animals (Annelida,

Arthropoda, Mollusca)• While considered less advanced than the

Deuterostomes, these phyla are the dominant animals on Earth today

Phylum Annelida Objectives

By the end of the lesson you should be able to:• State the advances over Nematodes• Describe all the body systems and structures

(specifically on the Earthworm)• Describe the 3 classes and their example animals

(and adaptations)

Phylum AnnelidaThe Segmented WormsCharacteristics and Advances:1. Their true coelom results in muscle around the

gut and more coordinated digestion. Specialization of the food tube begins

2. Nervous system has a single ventral nerve cord with an anterior brain

Phylum Annelida3. First phylum to have a circulatory system - frees

them from diffusion of materials - can grow larger. Also have hearts

4. Respiration through skin (land forms) by diffusion so must stay moist

5. Sexual reproduction (limited regeneration)6. Unspecialized segmentation separated by septa

Phylum Annelida7. Have

specialized excretory units called Nephridia in pairs in each segment

• Used as a kidney to get rid of metabolic wastes

Annelid ClassesThere are three major classes of Annelids:• Class Oligochaeta: the earthworm• Class Polychaeta: the sandworm• Class Hirudinea: the leech

Class Oligochaeta• Contains the

earthworm• Fairly well adapted to

land but must stay in moist environment

• Head is reduced as are obvious sense organs

• Move using setae

Class Polychaeta• Contains “parapodia”

that are modified setae• Used as paddles (thus

this class is marine) to move about - beginning of appendage development

• Also used as crude gills - beginning of respiratory system

• Include sandworms and tubeworms

Tube Worms

Cluster

close up of mouth

Class Polychaeta• The sandworm shows the

parapodia development• Compare the sandworm to the

centipede we’ll see later in the Arthropods

Class HirudineaLeeches• These are

ectoparasites that feed upon the blood of vertebrates including us.

• Most common in fish• Can be used for

medical purposesA leech in a persons eye