[Illustration: 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.]
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.