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~ 15 living species but a much more conspicuous part of the Cambrian fauna.  What taxon has been highly successful in replacing their niche (marine sediments), and why might this be?

• molting chitonous cuticle.

• eversible introvert on anterior end with spines around mouth (rings of scalids)  as in nemtamorphan larvae, kinorynchs, and locifera.

• body cavity large and with hemerythrin bearing blood cells (acts as circulatory system as in nematodes but no cells in nematodes)

• benthic, inhabiting marine sediments,  Typically carnivorous.

• The larval form (b) has a characteristic lorica (similar to that found in adult Lorcifera)  
  

Phylogeny

Priapulids were originally grouped with the echiuran polychaetes  and sipunculids.  Now, molecular sequence data and many morphological characteristics indicate a close relationship with other ecdyzoan, cycloneulia, and cephaloryhyncha (increasing closeness respectively).  What morphological characteristics support relationship with other ecdyzoan, cycloneulia, and cephaloryhyncha?   However, at least one cladistic analysis did not include the priapulids in ecdyzoan clade (Eernisse et al 1992).

It has been suggested that a priapulid ancestor with an armored cuticle may have given rise to arthropods (Conway Morris 2000).

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