That the development has in general followed those lines is made clear chiefly by the fact that the members of all these different orders of Crustaceans still arise from nauplius larvæ, even in those cases in which the perfect animal possesses a structure differing widely from the usual Crustacean form. All Crustaceans arise from the nauplius form, even those of the higher orders, though they may not arise from a nauplius larva. But this very circumstance, that in most of the higher and many of the lower Crustaceans, the young animal, when it emerges from the egg, already possesses more numerous segments and limbs than a nauplius larva, again points to the connexion between phylogeny and ontogeny, for in these cases the nauplius stage is gone through within the ovum. The whole difference between this and the forms we considered first lies in the fact that, in the latter, the development is greatly shortened, condensed, as we might say, so that the nauplius stage forms a part of the embryonic development, and that new segments and limbs develop in the embryo nauplius within the egg, so that the young animal leaves the egg in a more advanced state, nearer to that of the perfect animal, to which it can, therefore, attain in a shorter time.
Fig. 109. C, second Zoæa stage. The
thorax is now divided into cephalothorax
(Cph) and abdomen (Abd); seven pairs of
appendages are developed, and five more
(VIII-XII) are beginning to appear. Au,
paired eyes.
We should expect that this shortening of the larval period would be associated with a prolongation of embryogenesis, especially in those Crustaceans which possess a large number of segments and limbs, that is—in the higher forms—and in the main this is the case. But there are exceptions in two directions; in the first place there are some, even among the lower Crustaceans, which leave the egg not as a nauplius but in the perfect form of the adult, and secondly, there are, among the higher Crustaceans, certain species which emerge from the egg not in the more mature form but still in the primitive nauplius form. Fritz Müller was the first to furnish an example of this last case, a Brazilian shrimp, Peneus potimirim. Like the lowest Copepods or Branchiopods, this species, which belongs to the highest order of Crustaceans, goes through the whole long development, from the nauplius through a series of higher larval forms up to the perfect animal, and all outside of the egg, as an independent free-swimming larva (Fig. 109, A-E). This is in sharp contrast to its near relative, the freshwater crayfish, which goes through this whole development within the egg, and emerges perfectly formed.
We see from this example that it is not some inward necessity which thus, in the higher and more complicated organism, contracts the ontogeny into the embryonic state, but that this depends upon external adaptive factors. Here again we have adaptation, mainly to the conditions of larval life. The elimination of the larvæ by enemies, for instance, will, other things being equal, be so much the more incisive the longer the larval development is protracted, but in that case the general ratio of elimination of the species, and the degree of fertility the species must possess in order to hold its own in the struggle for existence, will also play a part in determining the mode of development. For the higher the ratio of elimination the more eggs the female must produce, and the more eggs that have to be produced the smaller will be the quantity of nutritive material for the building up of the young embryo which each egg can be furnished with. I know of no records in regard to the eggs of that Brazilian shrimp in which embryonic development ends with the nauplius stage, but we shall certainly not be wrong in predicting that the eggs in this case will be very small and very numerous, in contrast to those of the freshwater crayfish, which are large and, as compared with others known to us, not very numerous.
It is a point of undeniable theoretical significance which the life-histories of these Crustaceans disclose, that embryogenesis is not condensed according to hidden internal laws when the structure increases in complexity, but that the condensation of the ontogenetic stages depends upon adaptation, and may be quite different in nearly related species. It shows us anew that all biological occurrences are dominated by the process of selection.
Fig. 109. D, Mysis-stage. Thirteen pairs of appendages are now formed: I and II, antennæ; III, mandibles; IV and V, maxillæ; VI-XIII, swimming appendages with one branch or with two. Abd, abdomen. Sfl, tail-fin. E, the fully-formed Shrimp, with thirteen pairs of appendages on the cephalothorax (Cph); I and II, the two pairs of antennæ; then follow the maxillæ and maxillipedes (III-VIII), the last of which is visible in the figure, and the five pairs of walking-legs (IX-XIII) of which the third bears a long chela. On the abdomen there are now six pairs of appendages (XIV-XIX).
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).