Lecture Outline Ch. 9   Pseudocoelomate Animals - Spring 2003- L. Holmstrand

Pseudocoelomates include these Phyla:

        Phylum Rotifera

        Phylum Gastrotricha

        Phylum Nematoda

        Phylum Nematomorpha

Phylum Acanthocephala

Other minor Phyla not to be covered

New Feature - a body cavity

• 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 psuedocoelomates really related?

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

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
   

II. Selected  Pseudocoelomate Phyla

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

      1. Habitat - mostly freshwater planktonic and benthic forms

2. Morphology

   External features - Shape - fixed (lorica) or worm-like

- Body Regions

                       Head – corona

                Trunk

                 Foot and Toes – pedal glands         

          Internal features

                      Pharynx

                              Mastax

                              Trophi or “jaws”

            3. Maintenance Systems

               Feeding and nutrition  - filter feeders or predaceous      

Digestive System - complete and specialized

Excretion and Osmoregulation   - protonephridia with flame cells > bladder

- cloaca receives digestive, excretory & reproductive products

            4. Reproduction

               Rotifers are dioecious; sexual dimorphism (males small, rare, or absent)

   

Parthenogenesis (asexual) and Sexual reproduction both common

                       In some rotifer life cycles, Asexual & Sexual reproduction alternate.

• Parthenogenetic Cycle (Asexual)
  • 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 (2N) become dormant and overwinter
    • meiosis
    • 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 "tails"
• body with bristles, plates or spines

    3.   Characteristics:

• adhesive tubes
• browse on algae, protozoa, detritus
• complete digestive tract

   C.  Phylum Nematoda - Roundworms

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

"If all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable, and if, as disembodied spirits, we could investigate it, we should find its mountains, hills, vales, rivers, lakes and oceans represented by a thin film of nematodes." -N.A. Cobb, 1914

2. Important Facts about Nematodes

large phylum

variety of lifestyles, free-living and parasitic

     3.  Morphology 

        Cuticle - structure fibrous & stiff, with collagen; functions in chemical protection,

mechanical strength, locomotion   

"Skeleton" - muscles & fluid in pseudocoel

     5.  Nervous System - nerve cords and nerve ring

     6. Maintenance Systems 

Digestive System and feeding: Mouth - cuticle lined, sometimes toothed;

Muscular pharynx; Intestine with strong sphincters; pseudocoel under pressure

Excretion and Osmoregulation: secrete ammonia

7. Reproduction

• Dioecious
• Sexes dimorphic: distinguish male and female

8. Nematode Lifestyles

           Free-living

                Soil

                Water

                Vinegar "eels"

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

• Life Cycle:

  Adult - mammalian intestine

  Eggs shed in feces (can stay dormant for years)

  Mammals infected when eggs ingested

  Ingested eggs hatch - larvae migrate - bloodstream >lungs >trachea> pharynx>

   (swallowed again)

•       Effects - loss of energy, abdominal pain, tissue damage.
• 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 -has vestigial digestive tract; juveniles are endoparasites

of arthropods

4.      Life Cycle

Adults free-living - dioecious - females lay eggs

Larvae must find a host quick

       Juveniles endoparasites of arthropods - grasshoppers, beetles, 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

dioecious, internal fertilization

eggs hatch when eaten by host

juveniles are parasites of insects, other invertebrates