Worms and Mollusks

Phylum:Platyhelminthes

(flat) (worm) Simplest bilateral symmetric animals showing a head and tail region.

3 layers: endoderm, ectoderm, mesoderm

Known as acoelomates (without a coelom)

coelom : a fluid-filled body cavity

General body structure:

Feeding

Free-living flatworms can eat:Tiny aquatic animalsDead animalsSingle opening to function as a mouth and

anus.Possess a pharynx which is used to eat

Respiration, circulation, excretion

Breath through diffusion

Flame cells: specialized cells used for removing excess water.

Planarians (c. Turbellaria)

Free-living freshwater flatworm.Possess a digestive tract, mouth,

pharynx, and branched intestine.Feed on small microscopic animalsNo skeletal,circulatory,or

respiratory system

Planarians continued

Small brain with two nerve cords

HermaphroditesInternal

fertilizationCan also

reproduce asexually.

Marine Flatworms polycadiaStrange mating behaviorBeautiful colors

Parasitic flukes (c. Trematoda)Usually two or

more hosts. Primary host:

human (site of asexual reproduction)

Causes the disease schistosomiasis

Removal of a Liver Fluke…warning graphic

Tapeworms (c. Cestoda)

Parasites. Example: beef tapewormHead region (scolex) : contains suckers and

hooks used to attach to a host organism.Proglottids : square body segments used for

reproduction

Phylum: Nematoda (roundworms)

Long cylindrical bodiesOften covered with a thick cuticle1 mm to 1 meter in lengthBoth free-living and parasitic*** 2 body openings***

Major difference from the flatwormsFood and wastes will NOT be mixed

Form and function

Eat small animals, bacteria, algae, fungi, etc

Exchange gases through diffusion Possess a simple nervous system Move through contracting muscles Reproduce sexually

Trichina wormCauses trichinosisOften present in pigs. Grow to about one

millimeter in length and become cysts in pig muscles.

People eat contaminated pork and larval cysts develop into adults in human intestines.

New larvae can end up in human muscles.

Ways to prevent trichinosis:

1. Thoroughly cook pork products2. Feed hogs uncontaminated feed3. Freeze pork immediately after packaging

Filaria Roundworm that

causes the disease elephantiasis.

Carried by mosquitoes Infects bloodstream

and blocks lymph nodes resulting in severe swelling.

Ascarid worms

Affects more than 1 billion people

Affect people, horses, pigs, chicken, cattle, dogs, cats etc.

Pinworms and Hookworms

PINWORMSMost common nematode infection in N. AmericaEspecially infants and toddlers.

HOOKWORMSOften will enter

through the feet.Common in areas

where sewage disposal is inadequate

Phylum: AnnelidaAlso known as the segmented worms.The body is divided into separate body

segments.Marine,fresh water,and on land.Parasitic and free-living. ex.

Annelida Characteristics Characteristics of Annelida:-

1)Bilaterally symmetrical.

2)Body has more than two cell layers, tissues and organs.

3)Body cavity is a true coelom.

4)Body possesses a through gut with mouth and anus.

5)Body has 3 separate sections, a prosomium, a trunk and a pygidium.

6)Has a nervous system with an anterior nerve ring, ganglia and a ventral nerve chord.

7)Has a true closed circulatory system.

8)Has no true respiratory organs.

…more annelid facts“tube within a tube” body plan.Closed circulatory system, 2 openings

respirationAquatic annelids breathe using gillsLand-dwelling annelids breathe through the

skin

excretionNephridia : excretory organs

that filter nitrogenous waste in the ceolom.

ReproductionHermaphroditic. eggs and sperm released at

the clitellum

Marine Annelids

Class Polychaeta-means many bristles

C. OligochaetaFirst segment = prostomiumSetae = small hair-like extensions

on ventral surface used for locomotion.

Possess many “hearts”

The “hearts” of an earthworm:

Mainly freshwater.Suckers on both ends of bodySaliva of leeches contain enzymes

that prevent blood from clotting (anticoagulants)

Leech Anatomy

Attaches to prey with posterior sucker, and uses anterior sucker to “suck” blood.

Medicinal uses of leeches

Used to be used for “leeching” by early physicians to rid a sick person of “bad” blood.

Still used in surgery to prevent blood clotting and unwanted swelling due to excess blood.

Phylum: MolluscaSecond largest phyla of animals after the

arthropods.Found in fresh water, salt water, and on landex.

Mollusk characteristics:

Soft bodies, 3 cell tissue layersMany are used for food and jewelryPossess a foot = large ventral muscle used

for movement.Most have a radula (exception: bivalves)Mantle - fold of skin that surrounds the

body organs

1. bivalves2 parts to their shells (bi).Strong adductor muscles to keep the shell

closedIncurrent siphon – carries water and food

into the mantle cavityExcurrent siphon – siphon where water is

expelled.

Examples of bivalves:

pearls

Mother-of-pearl inner smooth part of bivlave shell.

Made primarily by oysters when a foreign object gets lodged inside between mantle and shell.

Come in many colors

Reproduction and nutrition

Bivalves are filter-feeders. (plankton and small invertebrates)

Separate sexesSperm is released by the excurrent

siphon and enters a female via the incurrent siphon where fertilization takes place.

Video of bivalve dissection

2. gastropodsLargest class of mollusksMost possess a single shell.exception: slug

3. cephalopodsMost advanced mollusksAll are marine predatorsUse tentacles to gather and manipulate foodCan swim rapidly by expelling a jet of

water from their mantle cavity.Some may discharge an inky fluid for

defenseOctopus (8 arms) squid (10 tentacles)

examples: