It is noteworthy that the males of parasitic Crustaceans are not only much smaller than the females (Fig. 113), but that they are also much less modified, and resemble the ancestral free-swimming Copepods to a much greater degree. They usually possess small but well-developed swimming-legs, and by means of these they seek out the female, dying after fertilization is accomplished. They are thus not sessile parasites at all, and have therefore to go through the stages of the free-swimming Copepods much more completely than the females, whose task is to accumulate within themselves from the blood of the fish as much material as possible for the forming of the eggs, and to produce the largest possible number of these. These therefore greatly surpass the free-swimming Copepods in fertility, as is evidenced by the enormous egg-sacs they bear at the posterior end of the body (Fig. 113, _ei_). Even among the higher Crustaceans, the so-called Malacostraca, the germinal history not infrequently exhibits more or less of the racial history in distinct recapitulation. [Illustration: FIG. 113. The two sexes of the parasitic Crustacean _Chondracanthus gibbosus_, enlarged about six times; after Claus. The main figure is that of the female, whose body bears quaint blunt processes. At its genital aperture (♂) a dwarf male is situated. _F_ and _F´_, the two pairs of appendages. _ei_, the long egg-sacs, portions of which have been cut off in the figure.] It is true however, as we have already shown, that there are only a few of the higher Crustaceans which emerge from the egg in the form of a nauplius; in most of them this stage has been shunted backwards in the ontogeny, and most of the crabs and hermit-crabs leave the egg in a higher larval form, that of the so-called Zoæa (Fig. 114). This term is applied to a larva which already exhibits two main divisions of the body, a head and thorax portion (cephalothorax, _Cph_) and an abdomen (_abd_). The cephalothorax is frequently equipped with remarkable long spines (_st_), and it always bears from five to eight pairs of limbs, anteriorly the antennæ (_I_ and _II_), then the mandibles (_III_), further back swimming-legs (_IV_, _V_), and behind these can be recognized the primordia of the other legs (_VI_-_XIII_), which will grow freely out later on. Large facetted and stalked eyes (_Au_) are borne on the head. This Zoæa form is not now found as a mature Crustacean form, so we cannot maintain with any confidence that it lived as a mature animal at an earlier period of the earth's history, but a second still more complex larval form of the higher Crustaceans is preserved for us in a group of marine Crustaceans, the Schizopods. These are Crustaceans which, though small, approach in external appearance our freshwater crayfish, only they have, instead of the ten walking-legs, biramose swimming-legs, by means of which they move freely in the water. The number of these branched legs is