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

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

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

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

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

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

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

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

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

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