these weapons of defence: and how complicated these defences must be! It has been calculated that more than ten thousand pieces, each admirably arranged and united, enter into the composition of the shell of the sea-urchin, to which no other can be compared. To abbreviate slightly Gosse's description of that wonderful piece of mechanism, the sea-urchin: "A globular hollow box has to be made, of some three inches in diameter, the walls of which shall be scarcely thicker than a wafer, formed of unyielding limestone, yet fitted to hold the soft tender parts of an animal which quite fills the cavity at all ages. But in infancy the animal is not so big as a pea, and it has to attain its adult dimensions. The box is never to be cast off or renewed; the same box must hold the infant and veteran urchin. The limestone can only increase in size by being deposited. Now the vascular tissues are within, and the particles they deposit must be on the interior walls. To thicken the walls from within leaves less room in the cavity; but what is wanted is _more_ room, ever more and more. The growing animal feels its tissues swelling day by day, by the assimilation of food. Its cry is, 'Give me space! a larger house, or I die!' How is this problem solved? Ah! there is no difficulty. The inexhaustible wisdom of the Creator has a beautiful contrivance for the emergency. The box is not made in one piece, nor in ten, nor a hundred. Six hundred distinct pieces go to make up the hollow case; all accurately fitted together, so that the perfect symmetry of the outline remains unbroken; and yet, thin as their substance is, they retain their relative positions with unchanging exactness, and the slight brittle box retains all requisite strength and firmness, for each of these pieces is enveloped by a layer of living flesh; a vascular tissue passes up between the joints, where one meets another, and spreads itself over the whole exterior surface." This being so, the glands of the investing tissue secrete lime from the sea water, and deposit it after a determinate and orderly pattern on every part of the surface. Thus the inner face, the outer face, and each side and angle of polyhedron, grow together, and the form characteristic of the individual is maintained with immutable mathematical precision. The dimensions and shape of these prickles are very variable. In certain Echinidæ they are three or four times the diameter of the body. In the urchin, properly so called, they are only three-fourths or four-fifths that diameter. They sometimes resemble short bristles. These defensive weapons have tubercles for supports, which are arranged on the surface of the animal with perfect regularity. At the base they present a small head separated by compression. This head is hollow on its lower face, presenting a cavity adapted to a tubercle of the shell. Each of the prickles, notwithstanding its extreme minuteness, is put in action by a muscular apparatus.