Lect Outline Ch.9 Pseudocoelomate Animals - Spring 2005 L. Holmstrand

The traditional grouping "Pseudocoelomates" included these Phyla: Rotifera, Gastrotricha, Nematoda, Nematomorpha, Acanthocephala and other minor phyla. Unifying features include small body size, body cavity a pseudocoelom, cutile, muscular pharynx and adhesive glands

NEW FEATURE - a body cavity

What is a "Pseudocoelom" (pseudocoel)?

persistent blastocyst of embryo
body cavity with one layer of mesoderm associated with body wall

What good is a (Pseudo)coelom?

1. Greater freedom of movement

2. Space for development and differentiation of organs

3. Means of circulation and distribution of materials throughout body

4. Storage place for waste products before excretion; Storage of gametes (primarily eggs)

5. A hydrostatic skeleton to facilitate movement

Compare exoskeleton, endoskeleton, hydrostatic skeleton
Hydrostatic skeleton involves circular & longitudinal muscles acting on a fluid-filled cavity

Phylogenetic Position - are pseudocoelomates really related?

Yes, through ____________________
No, according to ______________________
Maybe ...................

Molecular Phylogenetic studies divide protostomes into two major monophyletic groups:

LOPHOTROCHOZOA -

ECDYSOZOA -

Nematoda and Nematomorpha under this classification are placed in Ecdysozoa (not grouped with pseudocoelomates)

I. Pseudocoelomate Characteristics

triploblastic, bilateral symmetry
body wall of epidermis and muscle
Complete digestive tract
Circulatory & respiratory organs lacking
Some have primitive excretory systems- protonephridia
Eutely - cell constancy

· Sexes usually separate; resistant stages often present

II. Selected Pseudocoelomate Phyla

A. Phylum Rotifera "Wheel Bearing" or “Wheel Animal”- ciliated crown or corona

1. Habitat - mostly freshwater planktonic and benthic forms

2. Morphology

External features –

Shape - fixed (lorica) or worm-like

Three body regions - Head, Trunk, Foot and Toes

Internal features -

Pharynx

Mastax

Trophi or “jaws”

3. Maintenance Systems

Feeding and nutrition - filter feeders or predators

Digestive System - complete and specialized

Excretory System and Osmoregulation - protonephridia with flame cells > bladder >cloaca

- cloaca receives digestive, excretory & reproductive products

4. Reproduction

     Rotifers are dioecious; sexual dimorphism (males small, rare,

               or absent)

    Parthenogenesis common

In some rotifer life cycles (the Class Monogonanta), parthenogenesis and other types of sexual reproduction alternate.

Parthenogenetic Cycle:

Amictic eggs are diploid (2N) and hatch into amictic females

No meiosis; many generations

"Sexual" Cycle

Mictic eggs are haploid (N) and hatch into males if unfertilized

Fertilized eggs become dormant and can overwinter

Meiosis occurs; a few generations under suboptimal conditions

Distinguish between Amictic and Mictic Eggs

5. A "Gallery of Rotifers" - interesting and weird features of rotifers to show diversity

Rotaria – wormlike type; creeps and swims; uses corona for food gathering and locomotion

Testudinella – flat, fixed lorica (shell); well-developed muscles that retract the corona

Synchaeta – a small predatory rotifer; fast swimmer

Asplanchna – bag-shaped, crystal clear body; no gut; large embryo inside body

Brachionus – very common; carrying an egg; mass cultured as food for larval fishes

Kellicotia – lives in open water; very long spines; buoyancy; also deters predators

Keratella – lorica has spikes; spines on each corner; note cilia in tufts

Ptygura – sessile species which builds a tube of fecal pellets

Conochilus – spherical colonial species; gelatinous sphere in center

B. Phylum Gastrotricha - "hairy bellies"

1. Habitat - fresh and salt water

2. Morphology - small, ventrally flattened, with cilia

forked tail; body with bristles, plates or spines

3. Other Characteristics:

adhesive tubes

browse on algae, protozoa, detritus

complete digestive tract

4. Reproduction:

Marine species monoecious; reproduce sexually

Freshwater species - parthenogenesis; very large eggs; no larval stage; may lay

resting eggs

C. Phylum Nematoda - Roundworms

1. Habitat ...........almost everywhere

2. Important Facts about Nematodes

Huge phylum - 500,000? species

Variety of lifestyles:

Free-living -eat detritus, microbes, invertebrates; important decomposers

Parasitic - use plant and animal hosts - much damage, misery and cost

3. Morphology

Cuticle - 3 layers of crossed fibers, stiff, with collagen; functions in chemical protection, mechanical stength, locomotion

"Skeleton" - muscles & fluid in pseudocoel

4. Locomotion - body wall has only longitudinal fibers; no peristaltic movement

5. Nervous System - nerve cords and nerve ring

6. Maintenance Systems

Digestive System and feeding:

            Mouth is cuticle-lined, sometimes toothed or with stylets

Muscular pharynx; Intestine with strong sphincters; rectum cuticle lined;     pseudocoel under pressure

         Excretion: secrete ammonia; diffusion through body wall and through intestine into anus

          Osmoregulation: gland cells with lateral tubules empty into excretory pore; driven by                     hydrostatic pressure in pseucoel

7. Reproduction

Dioecious
Sexes dimorphic: distinguish male and female

8. Nematode Lifestyles

Free-living - soil, water, vegetation

An unusual habitat: Vinegar "eels"

A famous nematode: C. elegans

Parasitic Nematodes

infest plants, invertebrates, vertebrates

many in tropical parts of the world

EXAMPLES of Parasitic Nematodes

a. Ascaris lumbricoides - intestinal roundworm of vertebrates; feed on host's intestinal juices

Life Cycle:

          Adult - mammalian intestine

          Eggs shed in feces (can stay dormant for years)

          Mammals infected when eggs ingested

          Ingested eggs hatch - larvae migrate from bloodstream >lungs >trachea> pharynx> swallowed again

Effects of infection - loss of energy, abdominal pain, tissue damage.
Development

              Ascaris egg - thick, resistant - infective stage

              No free-swimming larval stage

              No intermediate host

b. Wucheria bancrofti - filarial worm

            Tropical - transmitted by blood sucking insects (intermediate host)

          Adult occupies lymph ducts and glands

Females release tiny microfilariae, juvenile worms

            Plugs lymphatic system - "elephantiasis"

Other filarial worms - "heart worm", "eye worm"

c. Dracunculus medinensis - Guinea worm

             Tropical

       Life Cycle

                        Larval stage in copepod in drinking water

                       Juvenile worm liberated in gut

Migrates to subcutaneous muscle for maturation and reproduction

When ready to lay eggs, crawl to near surface of skin

Breaks through skin to lay eggs

Eggs hatch into larvae, infect copepod; copepod ingested in unfiltered water

D. Phylum Nematomorpha "horsehair worm" "gordian worm"

1. Habitat - marine and freshwater

2. Morpholgy

long, resemble horse hairs

no distinct head; uniform diameter

thick cuticle

3. Feeding - adult does not feed; vestigial digestive tract; juveniles are endoparasites of arthropods

4.      Life Cycle

      Adults free-living; dioecious; females lay eggs

      Larvae must quickly find a host

      Juveniles are endoparasites of artropods - grasshoppers, crickets, etc

E. Phylum Acanthocephala "spiny-headed worms"

Endoparasites of vertebrate intestines (fish, birds, mammals)

1. Morphology

Proboscis covered with curved spines

Syncytial tegument

No digestive system

   2. Life Cycle

       Complex -2 hosts

       Dieocious - internal fertilization

       Eggs hatch when eaten by host

       Juveniles are parasities of insects, other invertebrates