Worms and Mollusks

Biology I: Chapter 27

FLATWORMS

Flatworms

• Phylum Platyhelminthes

• Soft, flattened worms

• Tissues and internal organs

• Simplest animals to have:

• 3 embryonic germ layers

• Bilateral symmetry (a right and a left)

• Cephalization

Flatworms

• Acoelomate: flatworms are without a coelom

• Coelom: fluid-filled body cavity lined with mesoderm

• The digestive cavity is the only body cavity

Feeding• Free-living flatworms

– Carnivores that feed on tiny aquatic animals

– Scavengers that feed on recently dead animals

• Parasitic flatworms

– Feed on blood, tissue fluids, or pieces of cells within a host’s body (Example: tapeworm)

Respiration, Circulation, & Excretion

• Rely on diffusion

• Flame cells: remove excess water and metabolic wastes from the body

Response

• Ganglia: group of nerve cells that controls the nervous system; in the head region

• Eyespot

Movement

• Cilia

• Muscle cells

Reproduction: Sexually and AsexuallyFree-living

• Sexually: hermaphrodites

– during sexual reproduction, two worms join in a pair, delivering sperm to each other

• Asexually: fission

Reproduction: Sexually and Asexually

Parasitic

• A complex life cycle including both sexual and asexual reproduction

Groups of Flatworms

• Turbellarians– Free-living; most live in marine or fresh water

• Flukes– Parasitic; infect the internal organs of their host

• Tapeworms– Long, flat, parasitic;

adapted to life inside the intestines of their host

ROUNDWORMS

Roundworms

• Phylum Nematoda

• Slender, unsegmented worm

• Pseudocoelom

• Digestive system with two openings: a mouth and an anus

Feeding

Free-living roundworms

• Carnivores that use grasping mouthparts and spines

Respiration, Circulation, & Excretion

• Exchange gases and excrete metabolic waste through their body walls

• No internal transport system

• Depend on diffusion

Response

• Simple nervous systems, consisting of several ganglia

• Run from the head to the tail

• Nerves transmit sensory information and control movement

Movement

• Hydrostatic skeleton

• Aquatic roundworms move like snakes

• Soil-dwelling roundworms push their way through by thrashing around

Reproduction

• Sexually

• Separate males and females

• Internal fertilization

• Male deposits sperm inside the female’s reproductive tract

• Parasitic roundworms have complex life cycles involving two or three different hosts or organs within a single host

Roundworms and Human Disease

Trichinosis-causing worms

• Adult worms live and mate in the intestines of their host (humans, pigs and other mammals)

Filarial worms

• Found primarily in tropical regions of Asia, threadlike worms that live in the blood and lymph vessels of birds and mammals, including humans, transmitted by biting insects, causes elephantiasis

Roundworms and Human Disease

Ascarid worms

• Serious parasite of humans and many other vertebrates, causes malnutrition; spread by eating vegetables or food that are not washed properly

Hookworms

• Hatch outside the body of the host and develop in the soil, can enter a barefoot and travel through the bloodstream to the intestines

Research on C. elegans

• Free-living roundworm Caenorhabditis elegans, C. elegans

• Feeds on rotting vegetation

• First multicellular animal whose DNA was fully sequenced

• Helps understand genes and how eukaryotes became multicellular

ANNELIDS

Annelids

• True coelom that is lined

• Segmented bodies

• Septa: internal walls between each segment

• Setae: bristles that are attached to each segment; used in respiration

Form and Function in Annelids

• Have complex organ systems

• Segmented body

Feeding and Digestion

• Filter feeders to predators

• Get their food using a pharynx

• Crop: in earthworms; part of the digestive system in which food can be stored

• Gizzard: in earthworms; part of the digestive system in which food is ground into smaller pieces

Circulation

• Closed circulatory system: blood is contained within a network of blood vessels

• Blood circulates through two major blood vessels that run from head to tail

Respiration

• Aquatic annelids: gills

• Land-dwelling annelids: diffusion through their moist skin

Excretion

• Two kinds of waste

• Digestive waste passes out through the anus at the end of the digestive tract

• Nephridia: excretory organs that filter fluid in the coelom

Response

• Well-developed nervous system consisting of a brain and several nerve cords

• The sense organs are best developed in free-living marine annelids

Movement

• Hydrostatic skeleton

• Longitudinal muscles and circular muscles

• Moves by alternating contracting these two sets of muscles

Reproduction• Sexually

• Some use external fertilization or have separate sexes

• Others, such as earthworms and leeches, are hermaphrodites

– Exchange sperm

– Clitellum: a band of thickened, specialized segments that secretes a mucus ring into which eggs and sperm are released and fertilization occurs

– The ring slips off the body and form a protective cocoon for the worms that hatch a week later

Groups of Annelids

• Oligochaetes– Streamlined bodies and have relatively few setae compared

to polychaetes, live in soil or fresh water

• Leeches– External parasites that suck the blood and body fluids of

their host

• Polychaetes– Marine annelids that have paired, paddlelike appendages

tipped with setae

Ecology of Annelids

• Provide passageways for plant roots and water and allow the growth of beneficial, oxygen requiring soil bacteria

• Important in the diet of many birds, moles, skunks, toads and snakes

• In the sea they participate in a wide range of food chains

MOLLUSKS

Mollusks• Phylum Mollusca

• Soft-bodied animals

• Internal or external shell

• Include snails, slugs, clams, squids and octopi

• Trochophore: free-swimming larval stage of an aquatic mollusk

Form and Function in Mollusks

• True coeloms

• Have complex organ systems

Body Plan

• Foot: muscular part of a mollusk

• Mantle: thin layer of tissue that covers most of a mollusk’s body

Body Plan

• Shell: structure in mollusks made by glands in the mantle that secrete calcium carbonate

• Visceral mass: area beneath the mantle of a mollusk that contains the internal organs

Feeding

• Herbivorous• Carnivores• Filter feeders• Detritivores• Parasites

• Radula: flexible, tongue-shaped structure used to capture food by snails and slugs

• Siphon: tube-like structure through which water enters and leaves the body, capturing plankton in the process

Respiration

• Gills inside their mantle cavity

• Land snails respire using a mantle cavity lined with blood vessels

• Typically live in moist places to keep this lining wet

Circulation

• Open circulatory system: blood is pumped through vessels by a simple heart

– Works well for slow-moving mollusks such as snails and clams (demands for oxygen are low)

• Closed circulatory system: can transport blood through an animal’s body much more quickly

Excretion

• Cells of the body release nitrogen-containing waste into the blood in the form of ammonia

• Nephridia remove ammonia from the blood and release it out of the body

Response

• Complexity of the nervous system varies greatly between mollusks

• Clams and other two-shelled mollusk lead inactive lives simple nervous system

Response

• Octopi and their relatives are active and intelligent predators most highly developed nervous system of all invertebrates

• Capable of complex behavior, such as opening a jar to get food inside

Movement

• Move in many different speeds

• Snails secrete mucus and move slowly over the surface using a rippling motion of the foot

• Octopus uses a form of jet propulsion, drawing water into its mantle and forcing it out the siphon

Reproduction

• Reproduce in many different ways

• Snails and two-shelled mollusk reproduce sexually by external fertilization

• Some mollusk are hermaphrodites

Groups of Mollusks

• Gastropods– Shell-less or single-shelled mollusks that move by using a

muscular foot located on the ventral side

• Bivalves– Have two shells that are held together by one or two

powerful muscles

• Cephalopods– Soft-bodied mollusks in which the head is attached to a

single foot; the foot is divided into tentacles or arms

Ecology of Mollusks

• Feed on plants, prey on animals, and clean up their environment by filtering algae out of the water or by eating detritus

• Filter-feeding bivalves can be used to monitor water quality

• Serve as subjects of biological research