1. PHYLUM ANNELIDA The Segmented Worms
Large phylum. There are about 9000 species of annelid known today. Sometimes called bristle worms because most annelids have small bristles (setae) that aid in locomotion. Earthworms and leeches most well known. About two thirds of the phylum are marine worms.

2. Annelids are true coelomates and are protostomes.

4. Characteristics Bilateral symmetry Body metamerically segmented
Chitinous setae (bristles) often present on parapodia
Digestive system complete
Respiratory gas exchange through skin, gills or parapodia
Closed circulatory system
Excretory system typically a pair of nephridia for each segment

5. Characteristics
Coelom well developed and divided by septa (except in leeches); coelomic fluid functions as hydrostatic skeleton
Coelom develops embryonically as a split in the mesoderm on each side of the gut (schizocoelomate)
Body wall with outer circular and inner longitudinal muscle layers
Transparent moist cuticle secreted by epithelium
Hermaphroditic or separate sexes
Larvae if present are trochophore
Asexual reproduction by budding in some
Spiral, determinate cleavage of the embryo

6. Body Plan 3 regions: Head – Prostomium & peristomium
Polychaete
Body Plan
3 regions:
Head – Prostomium & peristomium
Segments (metameres)
Pygidium
New segments formed at the posterior end
Annelids have three body regions. The majority of the body is comprised of repeated units called segments. Each segment is, in principle, limited by septa dividing it from neighbouring segments, and has a fluid-filled cavity within referred to as a coelom. Structures such as the excretory, locomotory and respiratory organs are generally repeated in each segment. Segments are formed sequentially in annelids and are established during development from growth zones located at the posterior end of the body; so the youngest segment in the body of an annelid is always the most posterior. The only parts of the annelid body that are not segmental are the head and a terminal post-segmental region called the pygidium.

7. Body plan Metamerism
Divides body into linear series of segments (metameres) separated by septa
Repetition of organs, tissues and appendages in each segment
Allows for regional specialisation

8. Body plan Metamerism Regional specialisation can result from:
Restriction of certain structures to only a few segments
Structures retained in all segments, but diverge and adopt different functions
Fusion of segments
Regional specialisation is not so important in the annelids, but will be seen when we deal with the arthropods.

9. Body plan Metamerism
Contraction of circular or longitudinal muscle causes a segment to become either longer and thinner or shorter and wider, respectively
Movement achieved by alternating waves of contraction of circular and longitudinal muscle (peristaltic contraction)
Segmentation is beneficial for locomotion as segments can be operated independently of each other. For example, one segment can be anchored to the ground while another is moved, which helps with burrowing.

11. Circulatory System Circulatory system closed
Respiratory pigments often present
Aortic arches (“hearts”) for pumping blood
Respiratory pigments are haemoglobin, hemerythrin or chlorocruorin.
Dorsal blood vessel – blood runs anteriorly
Ventral blood vessel – blood runs posteriorly
Five pairs of aortic arches (“hearts”) at position of esophagus – contract to pump blood from dorsal to ventral blood vessels

12. Nervous System Pair of cerebral ganglia (brain)
Two fused ventral nerve chords
Lateral nerves in each segment
Statocysts are balance organs

13. Nervous System
Giant axons in ventral chord conduct impulses very quickly
Sensory system of tactile organs, taste buds, statocysts (in some), photoreceptor cells and eyes with lenses (in some)
Statocysts are balance organs

14. Excretory System
Pair of nephridia in each metamere (except first 3 and last 1)

15. Excretory System Each nephridium occupies 2 adjacent segments
Nephrostome (internal opening) leads via small ciliated tubule to segment behind where it connects with main part of nephridium
Nephridium made of several loops of tubule that terminate in a nephridiopore (external opening)

16. Excretory System Cilia draw fluid from the coelom into the nephrostome
Water and salts reabsorbed
Urine released through the nephridiopore

17. Reproduction Monoecious or dioecious
Larvae (if present) are trochophore type
Trochophore: Free-swimming, translucent, marine larva with an ovoid body and ring of cilia
Asexual reproduction by budding in some
Trochophore = small, translucent, free-swimming larva characteristic of marine annelids and most groups of molluscs. Trochophores are spherical or pear-shaped and are girdled by a ring of cilia (minute hairlike structures) that enables them to swim.

18. Class: Polychaeta Largest class Mostly marine
Polychaete
Class: Polychaeta
Largest class
Mostly marine
May live under rocks or in crevices, burrow into sand, build tubes, or be pelagic
Well differentiated head with specialised sense organs
No clitellum
Free-moving
sedentary
The clitellum is a thickened glandular section of the body wall that secretes a viscid sac in which the eggs are deposited. Once the eggs have been deposited in the sac, the clitellum slides off of the earthworm's body.

19. Class: Polychaeta Paired appendages (parapodia) on most segments
Used for locomotion and respiration
Many setae, usually arranged in bundles on the parapodia

20. Class: Polychaeta Two groups: Sedentary polychaetes Errant polychaetes
Spend most of their time in tubes or permanent burrows
Ciliary feeders with tenticular crown
Errant polychaetes
Include free-moving pelagic forms, active burrowers and crawlers, and tube worms that leave their tubes for breeding and feeding
Predatory forms with pharynx, teeth and jaw
sedentary
Free-moving

21. Class: Polychaeta Reproduction:
prototroch
mouth
anus
Class: Polychaeta
Reproduction:
No permanent sex organs or ducts for sex cells
Sexes usually separate
External fertilization
Indirect development with trochophore larvae
Asexual budding in some

22. Class: Oligochaeta Earthworms
Mostly terrestrial or freshwater, some parasitic and a few live in marine or brackish water
Few setae per segment
No parapodia
Head absent

23. Class: Oligochaeta
Typhlosole increases surface area for absorption of nutrients

24. Reproduction in Earthworms
Hermaphroditic – copulate to exchange sperm
Copulating worms held together by mucous secreted by clitellum and by special ventral setae
Clitellum develops only in sexually mature worms
After copulation, each worm forms a cocoon around its clitellum, which slides forward along the body

26. Reproduction in Earthworms
As the cocoon moves along the body, eggs and sperm are released into it and fertilization takes place inside the cocoon
Cocoon eventually slides off the head of the worm and its ends close up
Young emerge from the cocoon as small worms- direct development

27. Class: Hirudinea Leeches Mostly freshwater, some marine or terrestrial
Dorsoventrally flattened
Usually both anterior and posterior sucker
No parapodia
Usually no setae
Some researches suggest that Oligochaeta and Hirudinea should be combined into one class, the Clitella.

28. Class: Hirudinea
Coelom filled with connective tissue and muscle – does not function well as hydrostatic skeleton
Many are carnivores of small invertebrates, some temporary parasites and some permanent parasites
Feed on body juices of prey – have a muscular pharynx and three jaws with teeth

29. Class: Hirudinea
Fixed number of segments (34) that are not usually divided internally by septa
Hermaphroditic
Clitellum appears only during breeding season
Direct development

30. Evolutionary Relationships

31. Evolutionary Relationships
Annelids most closely related to either arthropods or molluscs
Share with arthropods a segmented body plan and a similar nervous system
Share with molluscs similar features of early embryology and a trochophore larva