a) triploblastic organization (extensive mesoderm)*
b) complete gut (6 of 8 phyla)
c) small size (most < 5 mm; some as small as protists) associated with:

- eutely fixed cell number in many spp; one rotifer has 958 nuclei! one nematode (Caenorhabditis elegans) has 959 nuclei in adult; another (Rhabditis) has 200 nerve, 120 epidermal and 172 gut cells
- pseudocoelom or no fluid-filled body cavity (7 of 8 phyla)
- lack a blood-vascular system*
- protonephridial excretory system (7 of 8 phyla)

a) differs little from a hemocoel anatomically (partial or no epithelium separating fluid from tissues)
b) develops like a hemocoel in other protostomes except the gut is often free of mesodermal tissue
c) not associated with a pumping heart and thus functions mainly as a hydrostatic skeleton; in larger species it likely aids circulation

a) thin and flexible in some (e.g., rotifers), it is tough and must be molted to grow in others (many nematodes, nematomorphs, and 3 minor phyla)
b) when thin, it is associated with circular and longitudinal muscles, when thick it may antagonize longitudinal muscles
c) it lines the pharynx in many forms and may give rise to teeth around the mouth or body spines important in locomotion or feeding