Fig. 112. Development of the parasitic Crustacean Sacculina carcini, after Delage. A, Nauplius stage. Au, eye. I, II, III, the three pairs of appendages. B, Cypris-stage. VI-XI, the swimming appendages. C, mature animal (Sacc), attached to its host, the shore-crab (Carcinus mænus), with a feltwork of fine root-processes enveloping the crab's viscera. s, stalk. Sacc, body of the parasite. oe, aperture of the brood-cavity. Abd, abdomen of the crab with the anus (a).
From all this we may conclude that certain Cirrhipedes in times long past adopted a parasitic habit in the Cypris-larva stage, and that they gradually underwent adaptations to this mode of life, and that these went further and further, until the animal was transformed into the singular creature which we now see in the sexually mature form.
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.
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.
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 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.