I have already mentioned that exceptions to the usual mode of development occur even among the lower Crustaceans, and I was thinking at the time of the Daphnids, which leave the egg as fully formed little animals, already equipped with all their segments and limbs. The nauplius stage is passed through in the egg, and it is an interesting indication that the ancestors of the modern species were in the way of moulting, that this embryo nauplius moults within the egg by forming a fine cuticle which is shed after a time. If it be asked why there should be direct development in the case of these small and not very complex water-fleas, while related species, the Branchiopods, which are much richer in segments and in limbs, should emerge from the egg in the form of a nauplius, and then pass through a longer larval period, we may answer that the reason probably lies in the fact that, in the former case, very few eggs are produced, sometimes only one, often two, seldom more than a dozen, that these eggs can thus be relatively well equipped with yolk, and that the formation of the little body which bears only from seven to nine pairs of limbs can be easily completed within this egg. Other things being equal, the direct development would always be an advantage, because reproduction can begin sooner in the young generation and the number of individuals will thus increase more rapidly. And this is of particular importance in the case of the water-fleas. But if it be asked, further, why so few eggs are produced in this case, and whether these animals have no enemies, we must answer that, on the contrary, they are preyed upon and eaten in thousands by fishes and other freshwater animals, but that the drawback of the scanty production of eggs is counteracted on the one hand by their habit of reproducing parthenogenetically for the greater part of the year, and on the other hand by their habit of concealing the eggs in a special brood-chamber. This is the case not only in the summer eggs, to which nourishment is conveyed in the brood-chamber from the blood of the mother (Fig. 70), but also in the winter or 'lasting' eggs, which receive within the chamber a protecting covering (the shell or ephippium). [Illustration: FIG. 70 (repeated). Daphnella. _A_, summer ovum, with an oil-globule (_Oe_). _B_, winter ovum.]