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• 1) A grouping of convenience whose origins and membership remain controversial; features shared by most (or all*) phyla are:

  • a) triploblastic organization* with a complete gut (8 of 9 phyla)

  • b) small size (most <5mm) associated with: eutely (fixed cell number in many sp.), 'pseudocoelom' (7 of 9 phyla); lack blood-vascular system*, protonephridial excretory system (7 of 9 phyla)

  • c) determinate cleavage* with mesoderm from blastopore lip*

  • d) retractile or eversible spiny head end (6 of 9 phyla)

  • e) an acellular cuticle that must be molted (6 of 9 phyla)

• 2) A pseudocoelom differs little from a hemocoel (no epithelial lining):

  • a) it is not associated with a pumping heart, and thus functions mainly as a hydrostatic skeleton

  • b) it is not a persistent blastocoel, but develops just like a hemocoel in other protostomes (separation of connective tissue), and the gut is often free of mesodermal tissue

• 3) Most have a collagen or chitin cuticle that has many functions:

  • a) as a skeleton it may antagonize longitudinal muscles

  • b) it lines the pharynx in many forms and may give rise to teeth or spines important in locomotion or feeding

  • c) in some groups it is quite tough and must be molted to grow

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  Phylum NEMATODA

• 4) Phylum Nematoda (round worms)

  • a) may be most numerous of metazoans & 2nd most diverse phylum

  • b) most are free living, but many are also parasitic

  • c) cylindrical or spindle-shaped bodies surrounded by tough cuticle

  • d) large forms have an extensive pseudocoelom

  • e) body wall muscles are exclusively longitudinal

  • f) 4 nerve cords: dorsal & ventral (locomotion), 2 lateral (sensory)

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  Phylum ROTIFERA

• 5) Phylum Rotifera ("wheel bearer" worms)

  • a) among the smallest of metazoans, widespread in freshwater

  • b) huge variety of body forms & life cycles; often have a 3-part body (head, trunk, foot) with a ciliated crown (corona)

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