The Pseudocoelomate Body Plan:

Aschelminthes Chapter 11

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

orEcdysozoan (those that DO molt)

AschelminthesCharacteristics

–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(from ectoderm)

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

http://www.nobel.se/medicine/laureates/2002/press.html

Sydney Brenner- C. elegans since 1963

C. elegans Nobel Prize (2002)

Brenner, Sulston and Horvitz– “Genetic regulation of

organ development and programmed cell death”

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

LophotrochozoanPhylum 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 DigestionFeed 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 DigestionSome 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

FemalesGermovitellarium- ovary and vitellarium fuse

MalesSingle testisEversible penis-injects sperm into the pseudocoelom of female (hypodermic impregnation)

Divided into 3 classes:

Reproduction and Development

class

order

Class Seisonidea

Females produce haploid eggs that must be fertilized and develop into either males or females.

♀ & ♂ similar in size and formSexually 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 MonogonontaFemales 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.

Phylum Nematoda

The roundworms

Ecdysozoan

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 NematodaExternal 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 systemmouth->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

ReproductionSexual–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

Ascaris lumbricoidesAdults live in small intestines of

humansEggs 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 lumbricoidesWarning: 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

Necator americanusEggs hatch in warm moist soil

and releases a small larva, the larva molts and becomes the infective filariform larva.

hookworms living in soil

Necator americanusHumans become infected when

filariform penetrates the skin (usually b/w toes) to reach our circulatory system

Trichinella spiralisThe PorkwormAdults live in mucus of small

intestine of humans and other mammals

Adult female Adult male

Trichinella spiralisFemales birth young, larvae

enter circulatory system and are carried to Skeletal (striated) Muscles

Larvae entering into Skeletal Muscle cells

Trichinella spiralisLarvae encyst in muscles &

remains infective for many years

Another host must ingest infective meat to continue the life cycle

Trichinella spiralisHumans become infected by

eating improperly cooked pork products

Larvae encyst in stomach move to small intestine molt 4 times, turn into Adults

Wuchereria bancroftiThe Filarial wormsIn tropical countries over 250 million human infected

Wuchereria bancroftiThread-like worms that live in the

Lymphatic System, block the vesselsThis 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.

ElephantiasisWarning: Pictures not for

everyone!

Wuchereria bancroftiAdults 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 bancroftiMosquito (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 WormsDirofilaria immitis in US parasite of

dogsAdult worms live in heart, large

arteries, and lungsHeartworm 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