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 ganglioncerebral 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

SeptaSepta- Internal body walls separate the segments through which nutrients and other materials pass

Coelom-Coelom- Large, fluid-filled cavity located completely within mesoderm

Characteristics of AnnelidsOrgan systems include:

Closed circulatory systemExcretory structures called nephridianephridiaDifferent regions of gut for different functions

SetaeSetae- External bristles paired on each segment provide traction for crawling

Found on most

ParapodiaParapodia- Fleshy appendages on some

Annelid GroupsTwo characteristics used to classify Annelids:

Number of setae on each segmentPresence of parapodia

Marine WormsEarthwormsLeeches

Marine WormsClass 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 wormsNo parapodia and few setae on each segmentNo head region, nor eyesAortic archAortic arch- 5 tube-like heartsLight sensitive and touch sensitive organs at each end; sensory cells detect moistureConsume soil with pharynx which travels through one-way gutCrop-gizzard-intestine-anusWaste is called casting which fertilizes soil

EarthwormsFluid of coelem creates a hydrostatic skeletonhydrostatic skeletonEach segment has circular and longitudinal muscles which pull against this skeletonAlternately contract the two sets of muscles to moveClitellumClitellum- Contains male and female gametes; develops into cocoon after fertilization

Seminal vescicles- store earthworm’s own spermSeminal receptacles- Store the sperm of another worm

Class ClitellataSubclass 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