The same is the case with the numerous fish-parasites of the order Copepoda. They all leave the egg as nauplius larvæ, however greatly they may be modified later on by adaptation to a parasitic habit, and in them we can still observe, in the fully developed animals, a whole series of grades of transformation. Thus many genera, like _Ergasilus_, are distinguished from the free-swimming Copepods only by the modification of their jaws into piercing and sucking organs, and of a single pair of antennæ into hooks, by means of which they attach themselves to the fish on which they feed. In other genera the degeneration and modification go further; the antennæ, the eye, and the appendages degenerate more or less, and very remarkable attaching organs are sometimes developed, in the form of hooks or of knobbed pincers, or of actual suckers. In several types the degeneration and modification go so far that the segmentation of the body disappears, and the animal looks more like an intestinal worm than like a Crustacean (_Lernæocera_ and others). In all these forms adapted to a parasitic mode of life it is always only the mature animal which has been transformed in this manner, for previously it has gone through a series of stages which are quite similar to those of the free-swimming Copepods, beginning with the nauplius, and ending with the so-called Cyclops stage, that is, a larval form which possesses antennæ, eyes, and swimming-legs similar to our freshwater Copepods of the genus _Cyclops_. Here again we see in the ontogeny the repetition of a series of phyletic stages before the mature form is assumed. Why these stages should have persisted it is easy enough to understand, for how could an animal which emerged from the egg as a worm-shaped _Lernæocera_ find a fresh fish which would serve it as host? Yet these parasites could not possibly go on preying upon the same fish generation after generation. To secure the existence of the species it was therefore indispensable that the faculty of swimming should be retained at least in the young stages; in other words, that the free-swimming ancestral stages should be preserved in the ontogeny. In all these cases it is therefore beyond doubt that the germinal history recapitulates a series of stages comparable to those of the racial history, although not quite unchanged but adapted to the modern conditions of life, for instance in having shorter antennæ, smaller eyes, and with four instead of the usual five swimming-legs. The search for a host does not seem to last long, for fishes are usually found in large numbers together, and thus the young parasitic Crustacean does not require to make a long journey before it finds a refuge.