Few portions of comparative anatomy appear in a less satisfactory condition than that relating to the organs which have been classed together as glands without ducts. Since any attempt to illustrate the obscure physiology of these bodies must of necessity be founded on a precise knowledge of their distribution in the animal kingdom, the deficiency cannot be considered unimportant; and I accordingly venture to hope that the following details may not appear devoid of interest to the Royal Society. In recent researches I have had occasion to examine how far the presence of a thymus gland extends in the scale of organization: in the present paper I confine myself to the comparative anatomy of the thyroid body ; and I hope, on some future opportunity, to communicate the result of similar investigations in regard of the remaining two organs of the class, viz. the spleen and supra-renal capsule .