Chapter 27: Mollusks and Annelids

Section 2: Annelids

Annelids

· The soft-bodied earthworm is the most common terrestrial, or land-dwelling, segmented worm

· There are approximately ______species of segmented worms that live in moist soil, in fresh water, and in the sea

· Segmented worms, or ______, live just about everywhere in the world

What Is an Annelid?

· Phylum ______

· An annelid is a round, wormlike animal that has a long, segmented body

· Annelids range in size from tiny aquatic worms less than half a millimeter long to giant earthworms more than 3 meters long

· Annelids also vary greatly in color, patterning, number of bristles, and other superficial features

Form and Function in Annelids

· The many segments of an annelid’s body are separated by internal walls called ______

· Most of the body segments are virtually identical to one another

· However, some segments are modified to perform special functions

· For example, the first few segments may carry one or more pairs of ______, several pairs of ______, and other ______

______

Feeding

· The digestive tract extends from the ______to the ______

· Food enters through the mouth and travels through the gut, where it is ______

· Like mollusks, annelids have evolved structures and behaviors that allow them to use a wide variety of foods

· One feeding organ that has evolved many different forms in different groups of annelids is the ______, or the muscular front end of the digestive tube

· Many annelids can extend the pharynx through the mouth

· In carnivorous annelids, this type of pharynx usually has two or more ______attached to it

· When a suitable animal approaches, the worm lunges forward, rapidly extends the pharynx, and grabs the prey with its jaws

· When the pharynx returns to its normal position, it carries the food back to the ______

Respiration

· Aquatic annelids often breathe through ______

· Terrestrial annelids take in oxygen and give off carbon dioxide through their ______

· Because the skin must stay moist to make gas exchange possible, ______

______

· To help guard against this, terrestrial annelids, such as earthworms, secrete a thin protective coating called a ______to hold moisture around them

Internal Transport

· Annelids typically have ______organized around two blood vessels that run the length of their bodies

· In each body segment is a pair of smaller vessels called ______

______that connect the two main blood vessels and supply blood to the internal organs

· In annelids such as earthworms, several of the ring vessels near the anterior end of the worm are larger than the other ring vessels and have muscle tissue in their walls

o These vessels are often called ______because they contract rhythmically and help pump blood through the system

Excretion

· Annelids produce two kinds of wastes

o ______pass out through the anus at the end of the gut

o Wastes resulting from cellular metabolism are eliminated by ______

§ A pair of nephridia in each body segment removes waste products from the body fluids and carries them to the outside

Response

· Many annelids are active animals with ______

______

· The ______sits on top of the gut at the front end of the body

· Two large nerves pass around the gut and connect the brain with a pair of ganglia below

· From these ganglia, a ventral nerve cord runs the entire length of the worm

· Nerves from each segment of the worm enter and leave the nerve cord at a pair of small ganglia

· These nerves help carry messages from sense organs and coordinate the movements of muscles

· Sense organs are best developed in the ______

______of annelids

o Have sensory tentacles, statocysts, chemical receptors, and two or more pairs of eyes

· Many other annelids have much simpler sensory systems

o Earthworms have no specialized sense organs

o They rely on simple sensory cells in the skin

Movement

· Annelids have ______in their body walls

o ______muscles

§ Runs from the of the worm to the

§ When contracted, they make the worm ______

o ______muscles

§ Runs in circles around the body of the worm

§ When contracted, they make the worm ______

Reproduction

· Most annelids reproduce ______

· In some annelids, the sexes are separate

· However, annelids such as earthworms and leeches are hermaphrodites that undergo ______

· Although an individual worm produces both sperm and eggs, it rarely fertilizes its own eggs

· Instead, worms pair up, attach themselves to each other, and exchange sperm

· Each worm stores the sperm it has received in special sacs

· When eggs are ready for fertilization, a band of thickened, specialized segments called the ______secretes a mucus ring into which eggs and sperm are released

· The ring then slips off the worm’s body and forms a ______that shelters the eggs

Sandworms, Bloodworms, and Their Relatives

· Class ______

o Common and important marine worms

o Polychaetes are characterized by paired paddle like appendages on their body segments

o These appendages are tipped with ______

o Polychaetes live in cracks and crevices in coral reefs, in sand, mud, and poles of rocks, and even out in the open water

Earthworms and Their Relatives

· Class ______

o Contains earthworms and related species

o Oligochaetes are annelid worms that live in soil and open water

o Most oligochaetes live in soil or freshwater

o Oligochaetes have fewer bristles than polychaetes

Leeches

· Class ______

o Contains the ______, most of which live in tropical countries

o Freshwater organisms that exist as external ______, drinking blood and body fluids from their host

o All leeches have powerful suckers at both ends of their bodies

o These suckers are used to attach a leech to its host

· Leeches penetrate the skin of their host in one of two ways

o Use a muscular ______

§ Tubular organ that they force into the tissue of their host

o Use razor sharp ______

§ Once the wound has been made, the leech uses its muscular pharynx to suck blood from the area

· Both types of leeches release a special secretion from their salivary glands to prevent ______as they drink it

· Some leeches also produce a substance that ______the wound – thus keeping the host from knowing it has been bitten

· During feeding, a leech can swallow as much as ______its weight in blood

o Can take up to ______to digest

o A leech can live for a ______before it must feed again

How Annelids Fit into the World

· Important in many habitats

o Aquatic species are food for many fish, crab, and lobster

o Earthworms perform an essential task in ______

______

§ By constantly burrowing through the ground, they help ______the soil

§ Without the efforts of these annelids, the structure and fertility of farm soils would degenerate quickly, lowering ______

______