Key: Step 2

Elongated, oval or arrow shaped, with easily recognisable body- and head-part; head with 8 or 10 arms with suckers

Step 3

Head and body not clearly differentiated; head usually with 4 tentacles (may be 6, 2 or absent); body may have all kinds of protruding elements, but never sucker-bearing arms

Step 23

Worm-shaped; no arms or other protruding elements; body covered with calcified scales or spicula

Step 110
Decision path