The Pseudocoelomates

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pseudocoelom

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a fluid-filled cavity between the endoderm and ectoderm
-May have evolved before true coelom or after
- Differs from a true coelom because it is not entirely lined with mesoderm tissue
-Organs are not suspended or attached to membranes (mesenteries)

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Pseudocoelomate groups

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- Lophotrochozoa - lack a cuticle or don't shed their cuticle
- Ecdysozoa - shed a cuticle periodically
- both groups are bilaterally symmetric, triploblastic animals

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Phylum Nematoda
(Roundworms)
Basic Characteristics

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- V common, often locally abundant (5 billion in 1 acre of garden soil); important animals in the decomp process
- Slender, elongate, cylindrical body; tapered at both ends; round in cross-section
- outer cuticle usu. molted 4 times during maturation
- Two types of chemoreceptors(amphids anteriorly and phasmids posteriorly)
- aquatic have 2 ocelli
- only longitudinal muscles
- complete digestive system
- some have anterior w specialized feeding structures (lips, jaws, teeth or other)
- Anterior nerve ring w nerves extending from it

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cuticle

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multi-layered structure outside the epidermis of many invertebrates, in which it forms an exoskeleton

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Nematoda:
Osmoregulation

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- Free-living nematodes have renettes, ventral glands that absorb wastes from the pseudocoelomand empties them outside via a pore.
-Parasitic nematodes have a tubular system comprised of long canals with an excretory pore.

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Nematoda: Reproduction

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-Usually dioecious, sexually dimorphic (males smaller than females)
-Female reproductive structures paired; male has single testis
-Internal fertilization via copulation
-Some lay eggs (oviparous); others hatch eggs internally and give birth to larvae (ovoviviparous)
-Larvae resemble small adults

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Nematoda: Human Parasites

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-Have typical parasite adaptations (high reproductive capability, ability to find host, resistance to host’s enzymes)
Life cycle is not as complicated as in the Platyhelminthes, usually only 1 host
Some are annoying (e.g., pinworms); others cause severe problems (filarial worms)

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Phylum Nematophora:
Horsehair Worms

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WormsNematomorpha: General Characteristics
- Typically freshwater, but a few are marine or terrestrial
- Body is v elongated, wormlike
- Adults free-living; larvae parasitize arthropods (e.g., insects)
- Appear to arise spontaneously, sometimes in horse troughs (origin of name)

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Phylum Kinorhyncha: Kinorhynchs or Mud Dragons

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- marine sediment dwellers
- Feed primarily on diatoms and other food in sediment
- body of 11 segments
- spined cuticle helps w mvmt
- retractible head w cover plates called placids
- dioecious

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Phylum Priapulida: Priapulidworms or Penis Worms

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- marine sediment dwellers
- Feed primarily on material strained from sediment
- spiny, extensible proboscis
- 3 body segments: introvert (head), trunk, and caudal appendage
- Body ringed, often with spines on cuticle which aid in movement
- No specialized sensory organs
monoecious

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Phylum Loricifera: Loriciferans

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- Found attached to gravel in marine sediments
- Body covered by lorica (thickened cuticle)
- Possess retractable pharynx (introvert)
- Monoecious; deep sea species are parthenogenic
- Some adults appear to lack a pseudocoelom

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Phylum Acanthocephala:
Spiny-headed Worms

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- All are endoparasites of vertebrates (mainly fish)
- Retractible proboscis with recurved spines, used for attachment in host’s intestine
- No digestive system; absorb food through tegument
- Two hosts: intermediate host is an arthropod, definitive host is a vertebrate
- Dioecious w internal fertilization

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Phylum Rotifera:
Rotifers

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- v small, primarily marine
- some colonial, most free-swimming solitary
- Ciliated organs (CORONA) around the head used for food gathering & locomotion, which beat in a cycle, creating a wave that appears like a spinning wheel. hence the name
- Also have thickened cuticle called lorica
- head, trunk, foot body regions; foot secretes adhesive
- Pharynx has MASTAX, a specialized structure that grinds food (microorganisms and suspended organic material)
- complete GI, nervous system w brain, osmoregulation w protonephridia, sensory organs include eyespots & chemoreceptors

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Rotifera:
Reproduction

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- parthenogenesis
- In the Class Monogononta, 2 types of eggsare made, mictic and amictic. Amictic make only amictic females; mictic if unfertilized make males, if fertilized make amictic females. Fertilized mictic eggs overwinter in a dormancy.
- males usu have no/useless mouth, digestive organs

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sexual dimorphism

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sexes have differences in morphology. In invertebrates that usually means that the females are much larger than the males.

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Amphids and plasmids

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amphid = anterior
plasmid = posterior

- types of chemo receptors in Class Nematoda

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Renette gland

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In free-living Class Nematoda, ventral glands that absorb wastes from the pseudocoelom and empties them outside via a pore.

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Oviparous versus ovoviviparous

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- oviparous - lays eggs
- ovoviviparous - eggs hatched internally and birth larvae
- both appear in Class Nematoda