Chapter 27:Mollusks and Annelids

Section 1:

Mollusks

Mollusks

Phylum Mollusca Evolved in the sea more than 600 million years

ago More than 100,000 mollusk species

Divided into 7 classes Live everywhere Range in size from snails as small as a grain

of sand to giant squids that may grow more than 20 meters long

Wide range of forms and colors

Examples of Mollusks

What is a Mollusk?

Very different but all share similar developmental patterns Their different forms are the results of variations on the

same basic body plan Mollusks are defined as soft-bodied animals that have

an internal or external shell Although some present day mollusks lack shells, they

are thought to have evolved from shelled ancestors Most mollusks have a special kind of larva called a

trochophore Swim in open water and feed on tiny floating plants

Also seen in segmented worms Evolved from a common ancestor

Form and Function in Mollusks

Form and Function in Mollusks The body plan of almost all mollusks consists of four basic parts

Foot Usually contains the mouth and other structures

associated with feeding Many different shapes

Mantle Thin, delicate tissue layer that covers most of the body

Shell Made by glands in the mantle that secrete calcium

carbonate Visceral mass

Contains the internal organs

Form and Function in Mollusks Basic body parts have taken on different

forms as mollusks evolved adaptations to different habitats

Type of foot and shell mollusks have are used to group them into classes

Feeding

Every mode of feeding is seen in this phylum Many mollusks feed with a tongue-shaped

structure called a radula Layer of flexible skin that carries hundreds of

tiny teeth Inside is a rod of cartilage When the mollusk feeds, it places the tip of the

radula on the food and pulls the skin back and forth over the cartilage

Feeding

Feeding

Although they may have a radula, carnivorous mollusks such as octopi and certain sea slugs typically use sharp jaws to eat their prey Produce poisons

Mollusks such as clams, oysters, and scallops are filter feeders Use feathery gills to sift food from the water

Respiration

Gills serve as organs of respiration as well as filters for food

Aquatic mollusks breathe by using gills located inside their mantle cavities

Land snails and slugs breathe by using a specially adapted mantle cavity that is lined with many blood vessels

The surface is constantly kept moist so that oxygen can enter the cells Because the mantle loses water in dry air, most land

snails and slugs must live in moist places Prefer to move around at night, during rainstorms, and

times when humidity is high

Respiration

Internal Transport

Oxygen that is taken in by the respiratory system and nutrients that are the products of digestion are carried by the blood to all parts of a mollusk’s body

The blood is pumped by a simple heart through an open circulatory system Blood does not always travel inside blood vessels

Instead, blood works its way through body tissues in open spaces called sinuses

The flow of blood through sinuses is not efficient enough for fast-moving octopi and squids Closed circulatory system

Blood always moves inside vessels

Excretion

Mollusks must eliminate waste products Undigested food becomes solid waste that

leaves through the anus in the form of feces Cellular metabolism produces nitrogen-

containing waste in the form of ammonia Must be removed from body fluids

Simple tube-shaped organ called nephridia Remove ammonia from the blood and

release it to the outside

Response

Vary greatly in the complexity of their nervous systems Clams and other two-shelled mollusks

Simple nervous system Several small ganglia near mouth, a few nerve cords,

simple sense organs Octopi and other tentacled mollusks

Highly developed nervous systems Well-developed brain Complex sense organs Can be trained to perform different tasks in order to

obtain a reward or avoid punishment Often studied by psychologists interested in the way

animals learn

Reproduction

Sexes are separate and fertilization is external Most mollusks release eggs and sperm into

water Find each other by chance

Free-swimming larvae develop Tentacled mollusks fertilization takes place

inside the body of the female Some snails are hermaphrodites

Snails, Slugs, and Their Relatives Class Gastropoda

Gastropods “stomach-foot” Many have a one piece shell that protects their soft

bodies Some gastropods, such as slugs, have no shell

Protected by their behavior Some also contain toxins or chemicals that are

secreted to warn predators Bright colors warn predators to stay away

Snails, Slugs, and Their Relatives

Two-Shelled Mollusks

Class Bivalvia Shells with a hinge Common bivalves include clams, oysters, and scallops Although larvae are free-swimming, they soon settle down

to the relatively quiet life on the bottom of a body of water Most are sessile Mantle glands make the shells

Mantle glands also keep the shells inside surfaces smooth and comfortable by secreting layers of mother-of-pearl

If a grain of sand or small pebble gets caught between the mantle and the shell it forms a pearl

Two-Shelled Mollusks

Tentacled Mollusks

Cephalopods – members of the class Cephalopoda – are among the most active and interesting mollusks Includes octopi, squids, cuttlefish, and nautiluses

“head-foot” Most have 8 flexible tentacles equipped with a number of round

sucking disks that are used to grab their prey Move by a siphon and jet propulsion Can secrete large amounts of dark colored, foul tasting ink Can change color to to match their surroundings Most modern cephalopods have an internal shell or no shell at

all Contains gases that allow them to float

Tentacled Mollusks

How Mollusks Fit into the World Many different roles in living systems

Important source of food Environmental monitors Biological research Can cause harm to crops Cause sickness on occasion

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 9000 species of segmented worms that live in moist soil, in fresh water, and in the sea

Segmented worms, or annelids, live just about everywhere in the world

What Is an Annelid?

Phylum Annelida 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 septa 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 eyes, several pairs of antennae, and other sense organs

Feeding

The digestive tract extends from the mouth to the anus

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

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 pharynx, or the muscular front end of the digestive tube

Feeding

Many annelids can extend the pharynx through the mouth

In carnivorous annelids, this type of pharynx usually has two or more sharp jaws 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 gut

Many polychaete annelids, such as the sandworm Nereis, use hook like jaws to capture prey or nibble on algae

The digestive system of an earthworm is shown here. The pharynx pumps a mixture of food and soil into a tube called the esophagus. The food then moves through the crop, where it can be stored, and through the gizzard, where it is ground into smaller pieces. The food is digested in the intestine. Undigested materials pass through the intestine and are eliminated through the anus.

Respiration

Aquatic annelids often breathe through gills Terrestrial annelids take in oxygen and give

off carbon dioxide through their skin Because the skin must stay moist to make

gas exchange possible, the worms die if the skin dries out

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

The spaghetti worm uses its long tentacles to pluck bits of detritus from the ocean floor

In plume worms, a brush-shaped structure on the head is used in filter feeding and in respiration.

Internal Transport

Annelids typically have closed circulatory systems organized around two blood vessels that run the length of their bodies

In each body segment is a pair of smaller vessels called ring vessels 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 These vessels are often called hearts because they

contract rhythmically and help pump blood through the system

Excretion

Annelids produce two kinds of wastes Solid wastes pass out through the anus at

the end of the gut Wastes resulting from cellular metabolism

are eliminated by nephridia 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 well-developed nervous systems

The brain 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

Response

Sense organs are best developed in the free-living marine species of annelids Have sensory tentacles, statocysts, chemical

receptors, and two or more pairs of eyes Many other annelids have much simpler sensory

systems Earthworms have no specialized sense organs They rely on simple sensory cells in the skin

Sense organs are best developed in free-swimming annelids such as the paddleworm, which has a pair of beady eyes and a number of sensory tentacles on its head.

Movement

Annelids have two major groups of muscles in their body walls Longitudinal muscles

Runs from the front of the worm to the rear When contracted, they make the worm shorter

Circular muscles Runs in circles around the body of the worm When contracted, they make the worm skinnier

Reproduction

Most annelids reproduce sexually In some annelids, the sexes are separate However, annelids such as earthworms and

leeches are hermaphrodites that undergo internal fertilization

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

Reproduction

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 clitellum secretes a mucus ring into which eggs and sperm are released

The ring then slips off the worm’s body and forms a cocoon that shelters the eggs

Sandworms, Bloodworms, and Their Relatives Class Polychaeta

Common and important marine worms Polychaetes are characterized by paired

paddle like appendages on their body segments

These appendages are tipped with bristles Polychaetes live in cracks and crevices in

coral reefs, in sand, mud, and poles of rocks, and even out in the open water

Although they look very different from each other, both the fanworm and the fireworm are polychaetes. The fanworm is a filter feeder that retreats into its tube when threatened. The fireworm defends itself with poisonous bristles that break off and penetrate skin at the slightest touch. The pain caused by these bristles gives the fireworm its name.

Earthworms and Their Relatives Class Oligochaeta

Contains earthworms and related species Oligochaetes are annelid worms that live

in soil and open water Most oligochaetes live in soil or freshwater Oligochaetes have fewer bristles than

polychaetes

Leeches

Class Hirudinea Contains the leeches, most of which live in

tropical countries Freshwater organisms that exist as external

parasites, drinking blood and body fluids from their host

All leeches have powerful suckers at both ends of their bodies

These suckers are used to attach a leech to its host

Leeches

Leeches penetrate the skin of their host in one of two ways Use a muscular proboscis

Tubular organ that they force into the tissue of their host

Use razor sharp jaws Once the wound has been made, the

leech uses its muscular pharynx to suck blood from the area

Leeches

Both types of leeches release a special secretion from their salivary glands to prevent the blood from clotting as they drink it

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

During feeding, a leech can swallow as much as 10 times its weight in blood Can take up to 200 days to digest A leech can live for a year before it must feed

again

How Annelids Fit into the World Important in many habitats

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

Earthworms perform an essential task in conditioning soil By constantly burrowing through the ground, they

help aerate the soil Without the efforts of these annelids, the structure

and fertility of farm soils would degenerate quickly, lowering crop yields