A grouping of convenience; probably not monophyletic; tree & most characters from Nielson'94 p. 62, 235, 262.

        ==T==(placement uncertain)== Tardigrada (water bears)
        |
        |             ===3========== Rotifera (rotifers= "wheel bearers")
        |====2========|
<<=1====|             ===4========== Acanthocephala (spiny headed worms)
        |
        |   =============6========== Gastrotricha (gastrotrichs, hairy-belly worms)
        |   |
        =5==|         ===9========== Nematoda (round worms)
            |   ===8==|
            |   |     ==10========== Nematomorpha (horsehair worms)
            =7==|
                |     ==12========== Priapula (penis worms)
                ==11==|
                      |     ===14=== Kinorhyncha (spiny-snout propelled worms)
                      ==13==|
                            ===15=== Loricifera (corset-bearer worms)

T: aschelminth-like: a) pseudocoel, b) tri-radiate, myoepithelial sucking pharynx, c) cleavage not spiral or radial
1: a) triploblastic**, b) pseudocoelom, c) bilaterlly asymmetrical, determinate cleavage, d) mesoderm from blastopore rim, e) direct development without ciliated primary larva, f) protonephridia**
2: a) trunk with intracellular skeletal lamina (extracellular cuticle absent); b) sperm structure
3: a) toes with adhesive glands, b) one ventral nerve cord
4: a) loss of gut, b) apical proboscis with intracellular hooks, c) two lateral nerve cords
5: a) terminal mouth, b) radial pharynx, c) tripartite epipharyngeal brain
6: a) myoepithelial sucking pharynx, b) pseudocoelom lost, c) two lateral nerve cords
7: a) no locomotory cilia, b) cuticle molted, c) introvert with cuticular spines, teeth or scalids
8: a) collagenous cuticle, b) protonephridia lost, c) circular trunk muscles lost, d) non-ciliated sperm
9: a) 6+6+4 sensillae and amphids, b) four nerve cords (dorsal, ventral, 2 lateral)
10: a) adults lack gut, b) no amphids, c) one ventral nerve cord
11: a) chitinous cuticle, b) rings of scalids on introvert, c) one ventral nerve cord
12: a) large body cavity with amoebocytes and erythrocytes
13: a) mouth cone with cuticular ridges and spines, b) circular trunk muscles lost
14: a) segmented body (13 segments)
15: a) scalids with muscles, b) tri-radiate, myoepithelial sucking pharynx

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