1. Annelida: The Merametric Body Form
Chapter 13
Annelida: The Merametric Body Form

2. Phylum Annelida
Segmented worms believed to have evolved in the sea where two-thirds still live
Other one third are terrestrial
Fossils found up to 530 million years old
Range from 1 mm to 3 M long

3. Characteristics of Annelids
Each segment contains digestive, excretory, circulatory, and locomotor organs while some are modified for reproduction, feeding, or sensation
Anterior segment contains a cerebral ganglion (primitive brain) which is connected to a ventral nerve cord that runs along the underside of the body
Delivers sensory info. To brain

4. Characteristics of Annelids
Septa- Internal body walls separate the segments through which nutrients and other materials pass
Coelom- Large, fluid-filled cavity located completely within mesoderm

5. Characteristics of Annelids
Organ systems include:
Closed circulatory system
Excretory structures called nephridia
Different regions of gut for different functions
Setae- External bristles paired on each segment provide traction for crawling
Found on most
Parapodia- Fleshy appendages on some

6. Annelid Groups Two characteristics used to classify Annelids:
Number of setae on each segment
Presence of parapodia
Marine Worms
Earthworms
Leeches

7. Marine Worms Class Polychaeta- Parapodia with numerous setae
Largest group; live in ocean; eyes
Distinctive pair of paddle-like parapodia on each segment Ex: Nereis
Many burrow or are free-swimming, others live in protective tubes formed by gland secretions
Ex: Feather-dusters; tubes

8. Class Clitellata- No parapodia; few to no setae
Subclass Oligochaeta- Earthworms and some freshwater worms
No parapodia and few setae on each segment
No head region, nor eyes
Aortic arch- 5 tube-like hearts
Light sensitive and touch sensitive organs at each end; sensory cells detect moisture
Consume soil with pharynx which travels through one-way gut
Crop-gizzard-intestine-anus
Waste is called casting which fertilizes soil

9. Earthworms Fluid of coelem creates a hydrostatic skeleton
Each segment has circular and longitudinal muscles which pull against this skeleton
Alternately contract the two sets of muscles to move
Clitellum- Contains male and female gametes; develops into cocoon after fertilization
Seminal vescicles- store earthworm’s own sperm
Seminal receptacles- Store the sperm of another worm

10. Class Clitellata Subclass Hirudinea- Leeches eye spots
Lack setae and parapodia
Flattened body; segments not separated internally
Suckers at both ends of its body
Most species are predators or scavengers; some are parasites