The Pseudocoelomate Body Plan: Aschelminthes Chapter 11

Aschelminthes Seven different Phyla grouped into either the Lophotrochozoan (those that do NOT molt) or Ecdysozoan (those that DO molt)

Aschelminthes Characteristics First animal to posses a body cavity Pseudocoelom Distinct body cavity with no peritoneal lining and mesenteries. Visceral (internal) organs lie free in the cavity

A pseudocoelom Is a body cavity derived from the blastocoel, rather than from mesoderm Pseudocoelom Muscle layer (from mesoderm) Body covering (from ectoderm) Digestive tract Pseudocoelomates such as nematodes have a body cavity only partially lined by tissue derived from mesoderm.

Characteristics Pseudocoelom – acts as an internal hydrostatic skeleton that functions in locomotion

Characteristics Most have a complete tubular digestive tract (mouth-anus) First time mechanical breakdown of food, digestion, absorption, and feces formation

Characteristics Show eutely – same number of cells for each animal and for each given organ in all the animals of the species Ex. Caenorhabditis elegans (a type of nematode) has 959 cells Every worm in the species has 80 cells in their pharnyx

Nobel Prize (2002) Brenner, Sulston and Horvitz C. elegans Nobel Prize (2002) Brenner, Sulston and Horvitz “Genetic regulation of organ development and programmed cell death” Sydney Brenner - C. elegans since 1963 Using the nematode C. elegans this year's Nobel Laureates have demonstrated how organ development and programmed cell death are genetically regulated. They have identified key genes regulating programmed cell death and demonstrated that corresponding genes exist also in higher animals, including man. The figure schematically illustrates the cell lineage (top left) and the programmed cell death (below) in C. elegans. The fertilized egg cell undergoes a series of cell divisions leading to cell differentiation and cell specialization, eventually producing the adult organism (top right). In C. elegans, all cell divisions and differentiations are invariant, i.e. identical from individual to individual, which made it possible to construct a cell lineage for all cell divisions. During development, 1090 cells are generated, but precisely 131 of these cells are eliminated by programmed cell death. This results in an adult nematode (the hermaphrodite), composed of 959 somatic cells. http://www.nobel.se/medicine/laureates/2002/press.html

Characteristics Microscopic to meters in length Bilaterally symmetrical Unsegmented and cylindrical in cross section Most are freshwater Osmoregulation through protonephridia (mainly in freshwater)

Characteristics Triploblastic Most are dioecious No separate gas exchanges Some cephalization is present Cuticle present – may bear spines or scales and is useful for protection and taxonomic identification

Lophotrochozoan Phylum Rotifera (rota, wheel + fera, to bear) Small animals 0.1-3 mm Most abundant in freshwater About 1,000 cells (3 classes) and organs are eutelic

Phylum Rotifera All have a Corona (crown)- ciliated organ for locomotion and food gathering

Phylum Rotifera Posterior toes and adhesive glands called: Pedal gland- secretions aid in temporary attachment of foot to a substratum.

Feeding and Digestion Feed on small microorganisms and organic material Coronal cilia brings food to mouth Food enters the Pharynx that contains a muscular organ that grinds food called Mastax

Feeding and Digestion Then food passes through a ciliated Esophagus to the ciliated Stomach-complete extracellular digestion and absorption of food occurs.

Feeding and Digestion Some species have ciliated Intestine that becomes Cloacal bladder- receives water from the protonephridia, eggs from ovaries, and digestive waste. Cloacal bladder opens to the outside via an Anus

Other Organ Systems Protonephridia that empty into the cloacal bladder function in osmoregulation Nervous System- 2 lateral nerves and bilobed brain

Reproduction and Development Some reproduce sexually Many reproduce by parthenogenesis- a type of reproduction in which females produce offspring from unfertilized eggs

Reproduction and Development Females Germovitellarium- ovary and vitellarium fuse Males Single testis Eversible penis-injects sperm into the pseudocoelom of female (hypodermic impregnation)

Reproduction and Development Divided into 3 classes: class order

Class Seisonidea Females produce haploid eggs that must be fertilized and develop into either males or females. ♀ & ♂ similar in size and form Sexually reproduction only

Class Bdelloidea Females are parthenogenetic (asexual)- produce diploid eggs that hatch into diploid females.

Class Bdelloidea "The study of rotifers is a study of ladies, sometimes beautiful, often capricious, always fascinating.“ (Meglitsch)

Class Monogononta Females produce two kinds of eggs: Amictic eggs- diploid eggs that have not undergone reduction division, cannot be fertilized & develop only into females.

Class Monogononta Mictic eggs- undergone meiosis and are haploid- if: Unfertilized- develop quickly into males Fertilized- they secrete a thick shell and become dormant for several months before developing into amictic females.

Ecdysozoan Phylum Nematoda The roundworms

5 billion per acre

Phylum Nematoda Parasites and free-living Lack cilia except in their sensory structures Marine, freshwater, and soil habitats Triploblastic, bilaterally symmetrical Unsegmented vermiform (wormlike organism)

Phylum Nematoda External features Noncellular, collagenous cuticle Can molt 4 times during maturation Maintains internal hydrostatic pressure Mechanical protection Resists digestion by the host

Phylum Nematoda Longitudinal muscles are used for locomotion Thrashing movements (can’t crawl like worms)

Phylum Nematoda Sensory organs Amphids – chemoreceptors along the cuticle Phasmids - chemoreceptors near the anus Ocelli – eyespots found in aquatic nematodes

Feeding and Digestive System Carnivores, herbivores, omnivores, saprobes(decomposers), or parasitic Complete digestive system mouth->buccal cavity->pharynx ->tubular intestine->rectum->anus Hydrostatic pressure pushes food thru

Other Organ Systems Glandular system Aquatic nematodes Renettes – absorb nitrogenous waste Tubular system Parasitic nematodes Renettes form a canal

Other Organ Systems Nervous system Nerve ring from anterior to posterior Also have neuroendocrine secretions involved in growth, molting, cuticle formation, and metamorphisis

Reproduction Sexual Dioecious-having separate sexes Dimorphic-males are smaller than females internal fertilization

Reproduction Males One testis Bursa- used to insert their sperm into the female males have ameboid sperm

Reproduction Females Pair of convoluted ovaries Oviducts become uterus Several hundred to several hundred- thousand eggs per day Ovovivparity – giving birth to larvae that hatched from an egg

Some Important Nematode Parasites of Humans Ascaris lumbricoides Enterobius vermicularis Necator americanus Trichinella spiralis Wuchereria bancrofti

Ascaris lumbricoides The Giant Intestinal Roundworm- The adult female worm can be over 30 cm long and 2-6 mm wide 800 million infected- most common parasitic worm disease in the world the largest of the human intestinal nematodes. This is the most common parasitic worm disease in the world, very prevalent in tropical regions but rarely found also in our country. In the United States about 4 million people are infected. The adult female worm can be over 30 cm long and 2-6 mm wide. The female worm produces 200.000 eggs per day.

Ascaris lumbricoides Adults live in small intestines of humans Eggs exit with through feces Once the eggs are ingested, they hatch in intestine and travel to the lungs Larvae molt twice, they travel to the trachea where they are swallowed

Ascaris lumbricoides When Ascaris becomes a big problem. . . Warning: those with weak stomachs need not continue viewing. Only for the HBO audiences! When Ascaris becomes a big problem. . .

Enterobius vermicularis Pinworm Most common parasite in US Adults live in large intestine At night females migrate out of the anus and lay eggs on skin

Enterobius vermicularis Human ingest eggs, hatch and molt 4 times in small intestine & migrate to large

Necator americanus Found in Southern US Adults live in small intestine with teeth and feed on blood & tissue fluid Females 10,000 eggs daily & pass out of body in feces Hookworm

Necator americanus Eggs hatch in warm moist soil and releases a small larva, the larva molts and becomes the infective filariform larva. hookworms living in soil

Necator americanus Humans become infected when filariform penetrates the skin (usually b/w toes) to reach our circulatory system Outside defecation and walking barefoot maintains life cycle

Trichinella spiralis The Porkworm Adults live in mucus of small intestine of humans and other mammals Adult female Adult male

Trichinella spiralis Females birth young, larvae enter circulatory system and are carried to Skeletal (striated) Muscles Larvae entering into Skeletal Muscle cells

Trichinella spiralis Larvae encyst in muscles & remains infective for many years

Another host must ingest infective meat to continue the life cycle

Trichinella spiralis Humans become infected by eating improperly cooked pork products Larvae encyst in stomach move to small intestine molt 4 times, turn into Adults

Wuchereria bancrofti The Filarial worms In tropical countries over 250 million human infected Lymphatic vessels return tissue fluids to circulatory system

Wuchereria bancrofti Thread-like worms that live in the Lymphatic System, block the vessels This causes enlargement of various appendages: Elephantiasis An Adult female Wuchereria bancrofti is about 80-100 mm long and 0.24-0.30 mm in diameter, whereas a male is about 40 mm long and 0.1 mm in diameter.

Elephantiasis Warning: Pictures not for everyone!

Wuchereria bancrofti Adults copulate produce microfilariae. The microfilariae released into the blood stream A microfilaria is about 240-300 µm (micrometers) long and 7.5-10 µm thick

Wuchereria bancrofti Mosquito (intermediate host) feeds on human (definitive host) ingest microfilariae and larvae molts 2 times Mosquito bites another human it injects 3rd stage larvae into human blood, molts, enters lymphatic system

Other Filarial Worms Dirofilaria immitis in US parasite of dogs Adult worms live in heart, large arteries, and lungs Heartworm disease- fatal to dogs, unless give preventative medicine

Phylum Nematomorpha Phylum Nematomorpha Characteristics 250 species Horsehair worms Adults are free-living Juveniles are parasitic in arthropods No distinct head Dioecious