Zoology 250 Lecture 15
ASCHELMINTHES: DIVERSITY
(Aschelminthes study images)
- 1) PHYLUM NEMATODA (round worms) >15,000 named spp.; common in marine, freshwater & terrestrial systems
a) most are free living, but many are also parasitic
b) they are a group of many paradoxes:
- may be the most diverse phylum but only ~15,000 named spp.
- may be the most numerous metazoans on earth but few have seen them (most are very tiny, only parasitic forms get large)
- although few have seen them, some are serious human parasites (hookworm, pinworm, trichinella, filarial worm) & crop pests
- although few have seen them, one species (Coenorhabditis elegans) is one of the most intensively studied 'model' animals; the entire cell lineage of every cell in the adult body is known!
c) surprisingly little morphological diversity
- cylindrical or spindle-shaped bodies surrounded by tough cuticle
- have a mouth (with a muscular pumping pharynx) & straight gut
- body wall muscles are exclusively longitudinal (how do they move?)
- 4 nerve cords: dorsal & ventral (locomotory), 2 lateral (sensory)
d) large forms have an extensive pseudocoelom
- 2) PHYLUM ROTIFERA ("wheel bearer" worms) >2,000 spp.; mostly live in freshwater, but some marine species
a) are among the smallest of metazoans (most are < 0.5 mm; tiniest are <50 µm)
b) exhibit a huge variety of body forms & life cycles
c) often have a 3-part body (head, trunk, foot)
d) most are suspension feeders and feed with a ciliated crown (=corona)
e) some are predatory
f) some prey rotifers exhibit dramatic changes in body form in response to the scent of predators
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