Segmented worms

 Phylum Annelida is composed of segmented worms.  Worms having a body divided into many sections.  The name Annelida actually means “little rings”

 The 9,000+ species of annelids include earthworms, leeches, and a variety of marine worms. Marine Fireworm Leech Earthworm

 About two-thirds of all annelids are marine organisms.  Others are found in fresh water, and some are terrestrial.

 Annelids and Nematodes evolved around the same time from platyhelminthes and were the first animals to have a true coelom. Roundworms Earthworms Flatworm

 Formed entirely within the mesoderm, this kind of body cavity results in complex organs.

 Coelomate animals are found at the top of the phylogenetic tree of animals.  All are considered complex animals, but realize some are more advanced than others.

 Annelids were the first animals on Earth to have a body divided into segments.  Segmentation enables two main things: 1. Greater range of motion – the body is flexible 2. Improved body functions, or more specialization

 Mollusks are not segmented animals, but all other coelomate animals are.  Zoologists say that segmentation underlies the body plan of all higher coelomate animals.

 Another innovation that appeared during annelid evolution was paired appendages  Many of the marine annelids have a pair of fleshy lobes, called parapodia, that project from each segment

 These appendages function as swimming paddles and sites of oxygen exchange.  Many zoologists believe that parapodia are the beginnings of legs.

 All the organ systems are present in annelids.  The organs are suspended in the coelom by sheets of tissue

 Another feature of annelids is the short bristles called setae on the body segments of most annelids.  These tiny structures act as anchors and gripping devices.  The number of setae on the bodies of annelids or the absence of setae is a characteristic used to divide the phylum into classes

 The three main classes of Annelids are: 1. Class Polychaeta  Mostly marine forms with many bristles and usually brightly colored.  Chris Tree is an example

2. Class Oligochaeta  Mostly freshwater or terrestrial forms with few bristles.  Earthworms are examples

3. Class Hirudinea  No bristles  Leeches are examples