Pyloric caeca (and duct), of Asterias rubens, [439];

of Porcellanaster pacificus, [459];

absent in Hyphalaster moseri, [459]

Pyloric sac of Asterias rubens, [438]

Pyramids, building-stone of, [70]

Pyrenoid, [37], [115];

of Sphaerella, [126]

Pyrocystis, [110];

P. fusiformis, [132]

Pyrsonympha, [111];