Thus nature has in this case set bounds to the increase of a race which else might have destroyed the balance of marine existence; in most fishes, however, she has been obliged to provide against the danger of extinction by a prodigal abundance of new germs. If the cod did not annually produce more than nine millions of eggs, and the sturgeon more than seven; if the flat-fish, mackerels, and herrings, did not multiply by hundreds of thousands, they could not possibly maintain themselves against the vast number of their enemies. "Not one egg too much," every one will say who considers that of all the myriads of germs which are deposited on the shallow sand-banks and shores to be quickened by the fructifying warmth of the sun, not one in a hundred comes to life, as fishes and molluscs, crabs and radiata, devour the spawn with equal voracity; that a thousand dangers await the young defenceless fry, since everywhere in the oceanic realms no other right is known than that of the stronger; and that, finally, the insatiable rapacity of man is continually extirpating millions on millions of the full-grown fishes. But if very few of this much-persecuted race die a natural death, a life of liberty makes them some amends for their violent end. The tortured cart-horse or the imprisoned nightingale would, if they could reflect, willingly exchange their hard lot and joyless existence for the free life of the independent fish, who, from the greater simplicity of his structure, his want of higher sensibilities, his excellent digestion, and the more equal temperature of the element in which he lives, remains unmolested by many of the diseases to which the warm-blooded and particularly the domestic animals are subject. [Illustration: Dory.] CHAP. XIII. CRUSTACEA. CRABS--LOBSTERS. How are they distinguished from the Insects?--Barnacles and Acorn-shells.--Siphonostomata.--Entomostraca.--King-Crab. --Edriophthalmia.--Sandhoppers.--Thoracostraca.--Compound Eye of the higher Crustaceans.--Respiratory Apparatus of the Decapods.--Digestive Organs.--Chelæ or Pincers.--Distribution of Crabs.--Land Crabs.--The Calling Crab.--Modifications of the Legs in different species.--The Pinna and Pinnotheres.--Hermit Crabs.--The Lobster.--The Cocoa-nut Crab.--The Shrimp.--Moulting Process.--Metamorphoses of Crabs.--Victims and Enemies of the Crustaceans.--Their Fecundity.--Marine Spiders and Insects. [Illustration: Barnacle.] [Illustration: Balanus ovularis.] [Illustration: Development of Balanus balanoides.--(Acorn-shell.) A. Earliest form. B. Larva after second moult. C. Side view of the same. D. Stage immediately preceding the loss of activity. _a._ Stomach. _b._ Nucleus of future attachment.]