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46 the infected specimens. All, however, that have come under my notice were large females, and I have never seen a single Spæerularia in a worker or a male.

The worms lie free in the cavity of the body, and are somewhat curled up. The largest number of full-grown females which I ever found in a single bee was eleven, but the usual numbers were from five to eight. The two infected specimens of B. pratorum, however, contained only one specimen of the parasite apiece.

Von Siebold was quite correct in asserting that all the specimens observed by him were females; the males being, as mentioned below, very much smaller in size, and quite different inform and appearance. The full-grown females, as they are met with in May, June, and July, are nearly an inch long, more or less curled up, white in colour, sometimes opaque, sometimes more or less transparent, and of equal thickness from one end to the other, being everywhere about $1⁄15$th of an inch in diameter. The whole surface is covered with button-like projections, Pl. 1, Fig. 1, from which the very appropriate generic name is derived. These buttons are situated at equal distances from one another, and are of more or less equal size; each one is from $4⁄1000$ths to $7⁄1000$ths of an inch in diameter, and the intermediate spaces are a little smaller. There are, therefore, 10 longitudinal, and about 80 transverse rows, making, in all, about 800 of these projections; and each of them projects from $4⁄1000$ths to $6⁄1000$ths of an inch above the general surface of the body.

Generally these spherules are nearly as transparent as the rest of the skin; here and there, however, some of them are rendered quite opaque by the presence of innumerable, minute, greenish, elliptic bodies, each about $1⁄5200$th of an inch in length, by $1⁄10000$th of an inch in breadth. These darkened spherules are comparatively few in number, only one here and there being affected in this manner, except round the vulva, where from eleven to fourteen were generally in this condition. No other Nematoid worms have wart-like projections so much developed; many species, however, have, on particular parts, and especially in the male sex, buttons, much less than, but doubtless homologous with, those which are so much developed in Sphærularia, and have suggested for it a name so characteristic. Léon Dufour and Siebold considered the Sphærulari from the different sorts of humble bees, as belonging to one species; and all the specimens which have come under my notice have been very similar to one another, and have presented no differences of specific value. One specimen, however, was a little narrower than the rest, and more transparent; the buttons, also, were smaller than usual, and the body tapered a little towards the end which contains the vulva.

In turning to the internal anatomy, one can, with reference to some highly important organs, and systems of organs, only parody Van Troil's