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50 The young animals are born soon after the eggs are laid. They are about th of an inch in length, and th in diameter at the broadest part. They are very active; the skin has the appearance of being ringed. The head is pointed; the tail ends more abruptly, and makes a sudden curve. The anterior end of the body is transparent; but the rest is darkened by minute, round, strongly-refracting globules. As soon as the Humble Bees come out in spring, young Sphærulari may be found together with old ones, in some of them. I have met with them from the beginning of May till the middle of July, and the whole abdominal cavity of the humble bee often swarms with these little worms. In order to ascertain roughly what the number might be, I washed out the inside of a bee, and then collected all the young Sphærulari together. I then put them into a measuring bottle, and after shaking up, poured away half of the contents. Repeating this process, until only about a hundred were left, it was easy to calculate what the number must have been, if half had been removed a given number of times, though, of course, no great accuracy was thus obtainable. I repeated this experiment five times, and thence concluded that one specimen contained about fifty thousand young Sphærulari, three about sixty thousand, and one even over a hundred thousand! It seems almost inconceivable that a bee should live with such an immense number of parasites in its body; and still more so, that it should, meanwhile, go about its daily duties as if nothing was the matter.

These experiments, however, give but a faint idea of the number of young to which a single female Sphærularia might give birth. In every case the whole ovary was full of eggs, in various stages of development; and, considering the minuteness of the eggs, and the size of the ovary, the number present must be enormous. If the young worms can in any manner leave the bee without destroying it, there seems no reason why nearly all of these should not succesively come to maturity, and be hatched; but, even supposing that this is not the case, and that in the preceding experiment I have ascertained the greatest, or nearly the greatest number of young Sphærulari which can be produced in a single bee, still the chances against any one of them attaining to maturity must be very great; for it is evident that if the sexes of a given species are equal in number, and if the species is neither increasing nor diminishing, the chances against any given young one attaining to maturity may be obtained by halving the average number of young ones produced by each female.

It would seem, at first sight, that the history of the young Sphærularia was very simple. We might suppose that the infected bees would die in their nests; and that the young worms would then leave them, and immediately eat their way into other bees. This view would also be supported by the fact, that, at least as far as my experience goes, each infected bee contains, on an average, five or six Sphærulari. Two reasons, however, inconsistent though they may appear, militate against this supposition. The first is, that too large a proportion of the young Sphærulari would live; and the second is, that the