Phylum Annelida the segmented worms

Phylum Annelida •  Annelida from the Latin word annelus meaning “little ring”

•  All are segmented worms •  Approximately 15,000 species

including earthworms, freshwater worms, leeches, and marine worms

Annelida Characteristics

Triploblastic- 3 cell layers

Cephalization

Bilateral Symmetry

Highest level of organization: organs

Eucoelomate

Body Cavity Eucoelomate

Have a “true” body cavity that is completely surrounded by mesoderm

ectoderm

mesoderm

endoderm

coelom

Gut

Annelid Characteristics the coelom

ectoderm •  is a closed, fluid filled cavity that surrounds the gut •  the fluid within acts as a circulatory system (blood) •  tissue membranes (mesenteries) suspend organs in the coelom

Annelida Characteristics Metamerism

The body is made up of a series of repeating, coordinated segments called metameres that are separated from one another by septa.

• Each metamere contains sets of repeating organs

e.g. gut, blood vessels, nerve cord, excretory organs

Metamerism

septa

Metamerism

Annelid Characteristics •  Two part head consisting of

o  Prostomium: fleshy lobe that overhangs the mouth

o  Peristomium: first body segment

•  Annuli: circular rings •  Pygidium: the last segment; where the anus is •  Clitellum: reproductive structure

annuli

pygidium

Annelida Characteristics Nervous system

•  2 cerebral ganglia •  a ventral nerve cord with 2 ganglia per metamere.

•  In some species, sensory organs such as eyes, palps, and tentacles have arisen (palps are appendages that may or may not be covered in cilia that help the worm find and pull in food)

Annelida Characteristics Locomotion

•  both longitudinal and circular muscles which allow them to “wiggle”

•  most have setae (chitonous bristles secreted by the epidermis) that aid in locomotion and burrowing

Skeletal System •  fluid in coelom acts as a hydrostatic skeleton

Annelid Characteristics

Gas exchange •  mainly by diffusion through skin •  Class Polychaeta often has specialized structures for gas exchange (e.g. parapodia, gills)

Annelid Characteristics

•  unidirectional •  regional specialization (digestive organs)

Digestive System

Circulatory System •  closed circulatory system composed of blood vessels (some of which are contractile and act as “hearts”) •  some circulation is also accomplished by the coelomic fluid

Annelid Characteristics

•  excretion is accomplished by organs called nephridia (singular nephridium)

•  sexual

Excretion

Reproduction

Phylum Annelida

Class Polychaeta Class Oligochaeta Class Hirudinea

Class Polychaeta

Class Polychaeta

•  All marine •  This class contains 2/3 of all known Annelids •  Common species are clamworms, sandworms, and lugworms •  have a well developed head with specialized sense organs

Class Polychaeta

•  Commonly called “bristle worms” •  Have many setae (chitonous bristles secreted by the epidermis)

(Poly = many, chaeta= setae) •  These setae are arranged in bundles on paddle-like appendages called parapodia

setae

Class Polychaeta Parapodium

The parapodia function in gas exchange, locomotion, and feeding.

setae

Class Polychaeta

Tagmatization (tagmosis)

•  the fusion and specialization of formerly metameric segments

Class Polychaeta

Many are filter-feeders with specialized structures

Class Polychaeta

Many are predatory with specialized structures

Class Polychaeta

Many construct their own homes out of CaCO3 or sand debris and mucous

Class Polychaeta Reproduction

•  Usually dioecious (separate male and female) •  No permanent sex organs; gametes are shed into coelom where they stay until time for fertilization •  Fertilization is usually external in water •  Indirect development à trocophore larvae swims around until it grows into adult form by adding segments

Ecology •  Polychaetes often have effective defense strategies:

•  some have tubes to hide in •  some have vicious jaws •  some have modified “stinging” setae

a fireworm

Ecology

•  Some Polychaetes have a mutualistic relationship with their host

•  for example, many scaleworms are found near, or in the mouth, of brittlestars, starfish, and sea urchins. •  The scaleworm eats its host’s leftovers and with its vicious jaws, it will attack any predator trying to eat it’s host.

Class Oligochaeta

Class Oligochaeta

•  Habitats: terrestrial, freshwater, and marine •  Have few setae (Oligo = few, chaeta = setae)

Class Oligochaeta

What do earthworms eat?

•  Usually feed on detritus (decaying organic matter)

•  Depends on habitat, but they can feed on… Ø  Dead leaves and plant roots Ø  Living things such as other worms,

bacteria, and fungi Ø  Decomposing remains of other animals

Class Oligochaeta Do earthworms eat dirt?

•  Yes and No. •  Earthworms consume dirt as they burrow, so yes they do swallow it. •  HOWEVER, they do not get nutrients from the dirt. Their nutrients come from decaying plant and animal remains that are in the dirt.

Class Oligochaeta •  Earthworms have specialized digestive

system to obtain the maximum amount of nutrients out of the detritus.

•  Example: pharynx, crop, gizzard, etc.

Class Oligochaeta Locomotion

Circular muscle contraction

Longitudinal muscle contraction

Class Oligochaeta Reproduction

•  Usually monoecious •  Still have to have a partner. •  Cross-fertilize by exchanging sperm

clitellum

testis

Ecology

•  Earthworms are essential soil aerators, meaning they allow air to enter the soil. •  Mix the soil with their tunnels. •  Worm feces are great plant food = fertilizer!

Class Hirudinea

Class Hirudinea •  Includes leeches • usually freshwater but there are some marine and terrestrial species •  no septa between metameres •  no setae or parapodia •  have 2 suckers

Class Hirudinea

•  have an extendable proboscis for feeding

Class Hirudinea •  usually have a fixed number of segments (34) •  each metamere consists of several annuli (think accordion)

1 metamere

annuli

Class Hirudinea Locomotion

Lack septa between metameres, so they are incapable of moving like Oligochaetes.

Instead, they use their anterior and posterior suckers to move.

Class Hirudinea Reproduction

•  usually monoecious •  cross-fertilize by exchanging sperm

Ecology •  Although some leeches are parasitic blood suckers (can be temporary or permanent), many are predators.

Ecology

•  Leeches have been used medicinally since the 19th century.

•  Currently they are used to increase blood flow following reconstructive surgery

•  Hirudin is a powerful anticoagulant that is found in the salivary glands of leeches