• Formerly Aschelminthes, now divided into eight different phyla
• All phyla display traits that do not fit into the protostome or deuterostome line
• The pseudocoel, which previously united these phyla, has multiple developmental origins and varying size and importance
• We will consider four phyla: Rotifera, Acanthocephala, Nematoda, and Tardigrada

Advances in Aschelminths

1. Digestive - all have a mouth and anus

2. Circulatory

3. Nervous - brain, nerve cords, ganglia

4. Skeletal - pseudocoel - hydrostatic

5. Cuticle or thickened epidermis

6. Reproduction - dioecious, parthenogenesis

Phylum Rotifera

• "Wheel Bearers" from the feeding corona
• 1500 species, mostly freshwater
• May be quite abundant as lake zooplankton
• Some are attached or occur as interstitial fauna

Characteristics of Rotifera

1. Sack-shaped, elongate body, 0.1 to 1 mm

2. Anterior end bears ciliated organ - corona

3. The corona moves food into buccal tube:

buccal tube -> mastax -> intestine -> cloaca

4. May have well developed foot and toes (attachment and movement)

5. Lack a cuticle, but have a syncytial epidermis composed of actin filaments (some have a lorica)

6. Large pseudocoel, two pairs protonephridia

7. Nervous system: brain, antennal ganglia, paired nerve cords

8. Reproduction dioecious, parthenogenic

Rotifers: Life History and Ecology

• Parthenogenesis is quite important to the population biology of rotifers in lakes (seasonal peaks)
• Males have not been found in 90% of rotifer species
• In most species where males have been found, they are smaller with a degenerate digestive system
• In marine rotifers, males are always present with no sexual dimorphism
• Freshwater species capable of drying out
  • normal life span - a few weeks
  • in desiccated form - cryptobiosis - several years
  • cryptobiotic state may be widely spread by birds, wind, etc.
• May undergo cyclomorphosis
  • seasonal morphological change in response to predation
  • specific chemical cues
• Importance in the food webs of lakes
  • phytoplankton -> zooplankton -> planktivore -> piscivore
  • planktivorous fish prefer large zooplankton (crustaceans)
  • may increase the abundance of small zooplankton (rotifers)

Phylum Acanthocephala

• "spiny-headed" worms
• parasites of vertebrate gut tracts
• 1,200 species
• Terrestrial - insects (intermediate), usually birds and mammals (definitive)
• Aquatic - crustaceans (intermediate), usually fishes and amphibians (definitive)

Characteristics of Acanthocephala

1. Large pseudocoel, no digestive system

2. Spiny proboscis for attachment to host intestine

3. Dioecious - gonads suspended in pseudocoel by ligament sacs, fertilization is by copulation

4. Longitudinal retractor muscles

5. One pair of protonephridia open to gonopores (cloaca)

6. Fertilizated encapsulated egg develops into a spiny acanthor larva, which is eaten by intermediate host

7. Transmission to definitive host is by consumption of the infected intermediate host

Aschelminth Biology: Misfit Metazoans?

• Pseudocoelom
  • Larger aschelminths have pseudocoelom, smaller do not
  • If pseudocoelom is large, it is not from a persistent blastocoel
• Development
  • Protostomes? blastopore forms mouth, but cleavage irregular
  • Strongly determinate development
  • Many groups have a fixed number of cells - eutely
  • Rotifers <1000, Caenorhabditis has 1031
• Cryptobiosis
  • Ability to withstand extremely dry, cold conditions
  • Well-developed in nematodes, rotifers, tardigrades