Phylum Annelida

The Segmented WormsEarthworm

Leech

Nereis

Are There Many Kinds of Annelids??

• 15,000 species of annelids can be divided into three major groups

Polychaeta Oligocheata Hirudinea

What Are Annelids?General Characteristics

• Bilateral symmetry• Definate anterior /

posterior ends• Distinguishable dorsal /

ventral side• True tissue / organ level of

organization

• Segmented bodies• The distinguishing feature

of all annelids» Annelida = “little

rings” • Range in size

• < 1mm up to 3 meters!!» Eg. giant

Australian earthworm

The Internal Structure of

Annelids• Complete digestive

tract– Several specialized

regions

• Circulatory system– Closed vessels– Blood containing

hemoglobin

• Nervous system– Brain-like structure

• Reproductive structures– Ovaries and testes

All Annelids Are Coelomates . . .

• They have a true body cavity– Coelom is lined with

mesoderm cell layer

• “Tube-within-a-tube” body plan– Outside tube

• Body wall

– Inside tube• Digestive tract

Annelids are the simplest animals to have a true coelom!

What happens to the “stuff” annelids eat?!?

• Specialized regions aid in digesting the organic matter in soils ingested – CROP

• Temporarily stores food

– GIZZARD• Thick walled

organ which grinds food

Nereis . . . The Hunter• Marine Sandworm

– Found at tide level• Carnivorous

– Feeds on small animals• Distinct anterior segment

– Prostomium • 2 short tentacles + 2

palps• 2 pairs of small eyes

– Peristomium• 4 pairs of tentacles

surrounding mouth

Figure 32-14, Pg. 698

TentaclesPalps

How does the pharynx work??

• The pharynx is extended out of the mouth

• Pointed claw-like jaws capture prey

• Pharynx pulls food back through mouth

• Food moves into intestine where digestion begins

Jaw

Pharynx

These Guys Can Move!

• Nereis have structures allowing them to swim AND creep over sand

– Parapodia• Paddle-like extensions

– Setae• Bristle-like structures

Parapodia

Setae

Reproduction in Annelids• Oligochaeta &

Hurudinea– Hermaphrodites

• Cross-fertilization– Internal

fertilization

• Polycheata– Separate sexes– External

fertilization» Gametes are

released through excretory organs

» Free-swimming larva

Earthworms Create a Cocoon!

Clitellum : sperm are stored temporarily until a mucus cocoon is created to protect the fertilized eggs!

Leeches: the ecto-parasite!

• Feed on the blood of their prey

• Hirudin»Enzyme which

prevents blood clotting

• The jaws break through the host’s skin

• Blade-like jaws make incision

Two Evolutionary Innovations

• The presence of a true coelom– Provides support

» Acts as a hydrostatic skeleton

– Room for organ development– Advancements in locomotion

» Not the dance!!!! . . . Motility!!!

• Segmentation of the body– Specialization of body regions

» Feeding and sensory input, motility

– Modification of various segments for different functions

Worms today. . . Mollusks tomorrow!

Until then . . .

Be kind to your neighborly annelid . . .

You may depend on one someday!