Lecture 10: Annelids

Phylum Annelida

• segmented worms

• marine, freshwater, & terrestrial habitats

• metamerism

• paired setae (bristle like feet made of chitin)

metamerism

• repeating body segments

• coelom in segments separated by a wall called Septa

Hirudinae group

• ex: leeches

• have oral & posterior suckers

• no setae

• no septa between segments

• coelom full of connective tissue not fluid

• channels & sinuses are the circulatory system

Oligocheates group

soil burrowing worms

• no parapodia

• no suckers

• ex: earthworms

Polycheates group

marine worms with parapodia

• no suckers

class Errantia

free living worms w/ Polychaete body plan

Class Sedentaria

• sedentary (dont move much) and live in tubes/burrows

• polychaete body plan

• has the Order Clitellata

Order Clitellata

free living segemented worms with no parapodia

Annelida body structure labelled

segments are added in which area of the worm?

teloblastic growth zone (above anus)

• youngest segment infront of pygidium , oldest segment behind peristomium

Annelida internal body plan layers labelled

How do Annelids’ gut stay suspended in the center and form a hydrostatic skeleton?

Paired mesenteries hold the gut in the center by double lining the coelom.

• supports internal organs (viscera)

• fluid filled / constant volume

how do muscles control the movement of setae ?

1. formative cell = makes the seta

2. longitudinal & circular muscle = control body shape & movement

3. protractor muscle = push seta out to grip

4. retractor muscle = pulls seta back in

Locomotion of Leeches

1. anterior sucker detaches

2. circular muscles contract → body stretches forward

3. anterior sucker reattaches to surface

4. circular muscles relax, posterior sucker detaches

5. longitudinal muscles contract → body shortens, pulling the back end forward

6. posterior sucker reattaches

Errantia Polychaete Body plan labelled

What are the 3 speeds of locomotion for Polychaeta?

1. Slow crawling: mainly parapodia muscles

   • parapodia sides move in opposite direction

   • 6 - 8 segments used

2. Fast crawling: parapodia & body muscles used tg

   • wiggle side to side motion

   • 14 segments used

3. Swimming: parapodia & muscles used tg (more segments involved)

   • 40 segments used

Annelid circulatory system

closed

• dorsal vessel = carry blood forward

  • efferent vessels = carry oxygenated blood to the dorsal vessel

• ventral vessel = carry blood backward

  • afferent vessels = bring blood from ventral vessel to parapodia

• valves in vessels = keep blood flowing in one direction

• capillaries = tiny vessels where gas exchange happens between blood & tissues

  • parapodial vasculature capillaries = exchange gas between blood & outside environment

• circumesophageal vessels (hearts) = wrap around esophagus & help pump blood

what are the excretory organs of the adult annelid ?

Nephridia

• one pair in each segment (except 1st, 3rd, & last)

Nephridiopore is where the waste exits the body

what are excretory part of the adult annelid is the opening that sucks in the coelomic fluid ?

Nephrostome