even greater than ten, there are sixteen of them (Fig. 109 _D_, p. 164, _VI_-_XIII_). In the aquaria of the Zoological Station at Naples one may often see these dainty little creatures swimming about in large companies. Here they are of interest to us chiefly because their structure occurs in the ontogeny of the highest Crustaceans, the Decapods; that is, the phyletic stage represented by the Schizopods appears as an ontogenetic stage, just before the final metamorphosis of the larva to the perfect animal. This is the case in most of the marine Decapods, in those forms which do not go through the whole course of their development within the egg, but emerge as Zoæa larvæ, or even, as in _Peneus potimirim_, as nauplii. In the last-named species (Fig. 109) the ontogeny contains at least three stages which must have lived, perhaps not as mature forms, but as primitive larval forms, for unthinkable ages--the stage of the nauplus (Fig. 109 _A_), that of the Zoæa (Fig. 109 _B_ and _C_), and that of the Schizopod (Fig. 109, _D_); from this last the fully developed Decapod Crustacean arises (Fig. 109, _E_). We are, therefore, justified in saying that here the racial evolution is recapitulated in the individual development, although condensed and shortened in proportion as more numerous stages of the phyletic development are gone through within the egg, for there the different stages can succeed each other more rapidly and directly than in a metamorphosis of the free-swimming larvæ, since these must procure their own material for their further growth and their metamorphosis, while the yolk of the egg supplies a store of material which is sufficient for the production of a whole series of successive stages. [Illustration: FIG. 114. Zoæa-larva of a Crab, after R. Hertwig. _I_-_V_, the already functional anterior appendages--antennæ, mandibles, and swimming-legs. _VI_-_XIII_, rudiments of the posterior appendages of the cephalothorax (_Cph_). _Abd_, the abdomen. _st_, spine of the carapace. _Au_, eye. _H_, heart.]