Forcipate, [210], [211]

Formula, for Nematoda, [138]

Fornix, [481], [517], [525]

Fossil, Polychaeta, [301];

Polyzoa, [520];

Nemertinea, absence of, [119]

Fredericella, [476], [494], [502]-505, [518], [519];

lophophore, [495];

statoblast, [502], [503]

Frenzel, on Trematoda, [62];