We can thus understand why, among insects for instance, the male cells should always occur in the form of zoosperms, although in this case they reach a special receptacle in the female reproductive organs, the 'receptaculum seminis,' and are stored up in this. When a mature ovum gliding downwards through the oviduct comes to the place where this receptacle opens into it, the liberation of a few sperm-cells suffices to fertilize it with certainty, provided that they possess the thread-like form, which allows them to slip in through the very minute opening in the egg-envelope. It might be inferred from the certainty with which the ovum must in this case be found by the spermatozoon that only a small number of the latter would require to be produced, and yet even here the number is very large, though not so enormous as in the sea-urchins and other marine animals, which simply allow the sperm-cells to escape into the water. The large number in insects is due to the fact that many of the sperms may miss the micropyle; and also that in many insects a very large number of eggs have to be fertilized in succession. In the course of a life lasting three or four years the queen bee lays many thousand of eggs, most of which are fertilized, and that from a seminal receptacle which has been filled only once. There are, however, other sperm-cells of thread-like form which are not produced in such enormous multitudes, but in a much more moderate number, perhaps a few hundreds in the testicle. This is so in the little Crustaceans, known as Ostracods, all the freshwater species of which possess zoosperms only moderately numerous and of quite unusual size.