Chapter 27 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 animals Dead animals Single opening to function as a mouth and anus. Possess a pharynx

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 animals No skeletal,circulatory,or respiratory system

Planarians continued Small brain with two nerve cords Hermaphrodites Internal fertilization Can also reproduce asexually.

Parasitic flukes (c. Trematoda) Usually two or more hosts. (Page 687) Primary host: human (site of asexual reproduction) Causes the disease schistosomiasis

Tapeworms (c. Cestoda) Parasites. Example: beef tapeworm Head 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 bodies Often covered with a thick cuticle 1 mm to 1 meter in length Both free-living and parasitic *** 2 body openings*** Major difference from the flatworms Food 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 worm Causes trichinosis Often 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: Thoroughly cook pork products Feed hogs uncontaminated feed 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 Most common nematode infection in N. America Especially infants and toddlers.

HOOKWORMS Often will enter through the feet. Common in areas where sewage disposal is inadequate

Phylum: Annelida Also known as the segmented worms. The body is divided into separate body segments. Marine,fresh water,and on land. Parasitic and free-living. ex.

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

respiration Aquatic annelids breathe using gills Land-dwelling annelids breathe through the skin

excretion Nephridia : excretory organs that filter nitrogenous waste in the ceolom.

Reproduction Hermaphroditic. eggs and sperm released at the clitellum

First segment = prostomium Earthworms C. Oligochaeta First segment = prostomium Setae = small hair-like extensions on ventral surface used for locomotion. Possess many “hearts”

The “hearts” of an earthworm:

Ecology of Earthworms Help to aerate the soil Provide nutrients in to the soil in fecal matter Participate in a wide range of food chains

Suckers on both ends of body Leeches Mainly freshwater. Suckers on both ends of body Saliva 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: Mollusca Second largest phyla of animals after the arthropods. Found in fresh water, salt water, and on land ex.

Mollusk characteristics: Soft bodies, 3 cell tissue layers Many are used for food and jewelry Possess a foot = large ventral muscle used for movement. Most have a radula (exception: bivalves) Mantle - fold of skin that surrounds the body organs

1. bivalves 2 parts to their shells. Strong adductor muscles to keep the shell closed Incurrent siphon – carries water and food into the mantle cavity Excurrent siphon – siphon where water is expelled.

Examples of bivalves: clams and mussels scallops oysters

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 sexes Sperm is released by the excurrent siphon and enters a female via the incurrent siphon where fertilization takes place.

2. gastropods slugs conch snails Largest class of mollusks Most possess a single shell.exception: slug slugs conch snails

3. cephalopods Most advanced mollusks All are marine predators Use tentacles to gather and manipulate food Can swim rapidly by expelling a jet of water from their mantle cavity. Some may discharge an inky fluid for defense Octopus (8 arms) squid (10 tentacles)

examples: nautilus cuttlefish octopus squid