Worms and Mollusks Biology II: Chapter 27
FLATWORMS
Flatworms • Phylum Platyhelminthes
• Simplest animals to have:
• Soft, flattened worms
• 3 embryonic germ layers
• Tissues and internal organs • Bilateral symmetry (a right and a left) • Cephalization
Flatworms • Acoelomate: flatworms are without a coelom • Coelom: fluidfilled body cavity lined with mesoderm • The digestive cavity is the only body cavity
Feeding • Freeliving 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 Asexually Freeliving • 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 – Freeliving; 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 Freeliving 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 • Soildwelling 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 Trichinosiscausing 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 • Freeliving 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 • Landdwelling 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 • Welldeveloped 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 • Softbodied animals • Internal or external shell • Include snails, slugs, clams, squids and octopi • Trochophore: freeswimming 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, tongueshaped structure used to capture food by snails and slugs • Siphon: tubelike 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 slowmoving 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 twoshelled 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 twoshelled mollusk reproduce sexually by external fertilization • Some mollusk are hermaphrodites
Groups of Mollusks • Gastropods – Shellless or singleshelled 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 – Softbodied 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 • Filterfeeding bivalves can be used to monitor water quality • Serve as subjects of biological research