The facility with which the crustacea cast off their legs, and even their heavy claws, when they have been wounded in one of these organs or alarmed at thunder, is most remarkable. Without the least appearance of pain, they then continue to run along upon their remaining legs. After some time a new limb grows out of the old stump, but never attains the size of the original limb.
At the beginning of the chapter I have already briefly described the wonderful transformations of the barnacles, acorn-shells, and lerneæ, but the changes which the young crabs, lobsters, prawns, and shrimps, have to undergo ere they assume their perfect form are no less astonishing. Thus in the earliest state of the small edible crab (Carcinus mœnas) we find a creature with a preposterously large helmet-shaped head, ending behind in a long spine, and furnished in front with two monstrous sessile eyes like the windows of a lantern. By means of a long articulated tail the restless Chimera continually turns head over heels. Claws are wanting, and while the old crab is of course a perfect decapod, the young has only four bifid legs, armed at the extremity with four long bristles, that are continually pushing food towards the ciliated mouth. Who could imagine that a creature like this should ever change into a crab, to which it has not the least resemblance? But time does wonders. After the first change of skin the body assumes something like its permanent shape, the eyes become stalked, the claws are developed, and the legs resemble those of the crab, but the tail is still long, and the swimming habit has not yet been laid aside. At the next stage, while the little creature is still about the eighth of an inch in diameter, the crab form is at length completed, the abdomen folding in under the carapace. No wonder that these larvæ were long supposed to be distinct types, and described under the names of Zoëa and Megalops, until Mr. T. J. Thompson first discovered their real nature.
Metamorphosis of Carcinus mœnas.
A. First stage. B. Second stage. C. Third stage, in which it begins to assume the adult form. D. Perfect form.
Phyllosoma.
The life history of the Palinuri or spiny lobsters is equally curious. They frequently weigh ten or twelve pounds each, and are distinguished by the very large size of their lateral antennæ and by their feet being unarmed with pincers. Surely nothing can be more dissimilar than the glass crabs or Phyllosomas, thin as a leaf of paper, and so transparent that their blue eyes are their only visible parts while swimming in the water; and yet these flimsy creatures are nothing but the young of the large and bulky Palinuri.
Though several of the lower crustaceans ascend into the regions of eternal snow, while others hide themselves in the perpetual night of subterranean grottoes; though many delight in the sweet waters of the river or the lake, or rapidly multiply in stagnant pools, yet the chief seat of their class, which altogether comprises about 1,600 known species, is in the ocean and its littoral zone, where their numbers, their voracity, and their powerful claws, render them the most formidable enemies of all the lower aquatic animals that are not swift or cunning enough to escape them. Even the fishes and cetaceans are, as we have seen, exposed to their attacks; and as the whale, the carp, the sturgeon, the shark, the perch, have each of them their peculiar crustacean parasites, it can easily be imagined how large the number of still unknown species must be which feast on that vast host of fishes that has never yet been accurately examined. On the other hand, the crustaceans constitute a great part of the food, as well of the sea-stars, sea-urchins, annelides, and many of the molluscs, as also of the fishes and sea-birds; and as they are found of all sizes, from microscopical minuteness to the gigantic proportions of the Inachus Kæmpferi of Japan, the fore-arm of which measures four feet in length, and the others in proportion, so that it covers about 25 feet square of ground, they are able to satisfy the wants or the voracity of a vast number of enemies, from the rotifer or the polyp that feed on tiny entomostraca or the larvæ of the barnacle, to man, who selects a great variety of the fat and luscious decapods for his share of the feast.
A great fecundity enables the crustaceans to bear up against all these persecutions. 12,000 eggs have been found on the lobster; 6,807 on the shrimp; 21,699 on the great crab (Platycarcinus pagurus). The lower orders are still more prolific, for such is the rapidity with which many of them come to maturity and begin to propagate that it has been calculated that a single female Cyclops may be the progenitor in one year of 4,442,189,120 young! Endowed with such powers, the crustaceans are not likely to be extirpated, nor to disappoint the hopes of their gastronomical admirers for many an age to come.