Auditory organs, of Turbellaria, [26];

of Hoplonemertea, [106], [110];

of Nematoda, [128];

of Polychaeta, [273]

Aulastomum, [393], [399], [403]

Auricles, of Rotifers, [205]

Autolytus, [308];

eye, [255];

denticles, [270];

brood-sac, [275], [276];