a) Ph. NEMATOMORPHA (horsehair worms; 325 sp.)- terrestrial, freshwater; semiparasitic
- extremely long, slender worms resembling nematodes
- single large pseudocoelom, sometimes filled with mesenchyme
- like nematodes, they lack circular muscles
- larvae are parasitic in terrestrial insects; they induce the host to move to fresh water, where the typically non-feeding adults emerge to reproduce
- only the larvae have a spiny, eversible head end (introvert)
b) Ph. PRIAPULIDA (penis worms; 18 spp.)- marine, bottom dwelling, free living
- thick-bodied, can be large (0.5 - 40 cm long), typically predatory worms with a large, spiny, eversible head end (introvert)
- distinctive, plumose caudal appenge (gill?) in some
- single large pseudocoelom
- although few living species persist, they were a common invertebrate of Cambrian seas and fossils are well-known
c) Ph. ACANTHOCEPHELA (spiny-headed worms; 1150 spp.)- freshwater parasites
- small-bodied (typically 1-2cm), exclusively endoparasitic
- require two hosts (arthropod intermediate, vertebrate final host)
- no mouth or gut
- single large pseudocoelom
- spiny, eversible head end (introvert) to anchor in the gut lining
- related to rotifers: share syncytial epidermis, anterior flagella on sperm, similar 18S rRNA sequence