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 (Caenorhabditis elegans) is one of the most intensively studied 'model' animals; the entire cell lineage of every cell in the adult body is known!
c) they exhibit 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 microns!; most exhibit eutely) 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