"And so I live, you see, Go through the world, try, prove, reject, Prefer, still struggling to effect My warfare; happy that I can Be crossed and thwarted as a man, Not left in God's contempt apart, With ghastly smooth life, dead at heart, Tame in earth's paddock as her prize. ... ... ... ... ... Thank God, no paradise stands barred To entry, and I find it hard To be a Christian, as I said."--_Browning._
"Work out your own salvation."--_Paul._
"Be no longer a chaos, but a World, or even Worldkin. Produce! Produce! Were it but the pitifullest infinitesimal fraction of a Product, produce it, in God's name!"--_Carlyle._
From a study of the habits and organization of the family of Hermit-crabs we have already gained some insight into the nature and effects of parasitism. But the Hermit-crab, be it remembered, is in no real sense a parasite. And before we can apply the general principle further we must address ourselves briefly to the examination of a true case of parasitism.
We have not far to seek. Within the body of the Hermit-crab a minute organism may frequently be discovered resembling, when magnified, a miniature kidney-bean. A bunch of root-like processes hangs from one side, and the extremities of these are seen to ramify in delicate films through the living tissues of the crab. This simple organism is known to the naturalist as a Sacculina; and though a full-grown animal, it consists of no more parts than those just named. Not a trace of structure is to be detected within this rude and all but inanimate frame; it possesses neither legs, nor eyes, nor mouth, nor throat, nor stomach, nor any other organs, external or internal. This Sacculina is a typical parasite. By means of its twining and theftuous roots it imbibes automatically its nourishment ready-prepared from the body of the crab. It boards indeed entirely at the expense of its host, who supplies it liberally with food and shelter and everything else it wants. So far as the result to itself is concerned this arrangement may seem at first sight satisfactory enough; but when we inquire into the life history of this small creature we unearth a career of degeneracy all but unparalleled in nature.
The most certain clue to what nature meant any animal to become is to be learned from its embryology. Let us, therefore, examine for a moment the earliest positive stage in the development of the Sacculina. When the embryo first makes its appearance it bears not the remotest resemblance to the adult animal. A different name even is given to it by the biologist, who knows it at this period as a Nauplius. This minute organism has an oval body, supplied with six well-jointed feet by means of which it paddles briskly through the water. For a time it leads an active and independent life, industriously securing its own food and escaping enemies by its own gallantry. But soon a change takes place. The hereditary taint of parasitism is in its blood, and it proceeds to adapt itself to the pauper habits of its race. The tiny body first doubles in upon itself, and from the two front limbs elongated filaments protrude. Its four hind limbs entirely disappear, and twelve short-forked swimming organs temporarily take their place. Thus strangely metamorphosed the Sacculina sets out in search of a suitable host, and in an evil hour, by that fate which is always ready to accommodate the transgressor, is thrown into the company of the Hermit-crab. With its two filamentary processes--which afterward develop into the root-like organs--it penetrates the body; the sac-like form is gradually assumed; the whole of the swimming feet drop off--they will never be needed again--and the animal settles down for the rest of its life as a parasite.
One reason which makes a zoologist certain that the Sacculina is a degenerate type is, that in almost all other instances of animals which begin life in the Nauplius-form--and there are several--the Nauplius develops through higher and higher stages, and arrives finally at the high perfection displayed by the shrimp, lobster, crab, and other crustaceans. But instead of rising to its opportunities, the sacculine Nauplius having reached a certain point turned back. It shrunk from the struggle for life, and beginning probably by seeking shelter from its host went on to demand its food; and so falling from bad to worse, became in time an entire dependant.