Phylum AnnelidaPhylum AnnelidaThe Segmented WormsThe Segmented Worms

• Annelids are to be found in marine, freshwater, and terrestrial habitats

• The body is vermiform, bilaterally symmetrical, and segmented

• The body is triploblastic with a well developed coelom

Classes of Annelida

• Class Polychaeta (fanworms, clam worms)• Class Oligochaeta (earthworms)• Class Hirudinea (Leeches)

• Classes not covered– Class Branchiobdellida– Class Acanthobdellida

General Characteristics

• Each segment is separated from contiguous ones by a transverse septum

• The body is covered with a flexible non-chitinous cuticle• Respiratory gas exchange through skin, gills, or

parapodia• Most forms have chaeta, hard, bristle-like chitinous

structures• Excretory system typically consists of a pair of

metanephridia per segment

Class - PolychaetaFeather Duster worms, Clam worms, Christmas Tree worms

• Mostly marine and free-living• Many chaeta, on fleshy lateral outgrowths of the

body wall known as parapodia• Well developed head bearing appendages• Sexes separate, with a free-swimming

trochophore larva• No clitellum

Figure 17.03d

Class OligochaetaEarthworms and Freshwater Worms

• Are terrestrial with a few freshwater species• Have few chaeta• No parapodia• No distinct head appendages• Clitellum present• Hermaphrodites, with copulation required• Eggs are deposited in a cocoon and development is

direct

Earthworm Dissection

Figure 17.12c

Figure 17.15

Class Hirudinea

• Ectoparasites, not all - many feed on organic debris• Usually dorso-ventrally flattened with fixed number

of body segments (34) • No chaeta or parapodia• Clitellum present only during reproduction• Reproduction similar to earth worms/direct

development• Two suckers – anterior and posterior

Figure 17.20• Coelom reduced by invasion of connective tissue