They are furnished with two suckers, one at each end, and a mouth with numerous fine teeth placed in a tri-radiate manner, with which they pierce the skin when about to feed. They move in the water with great rapidity by a wavy motion of their bodies, but on solid substances their mode of progression is that of adhering by the sucker in front, then drawing the other one close up and extending the body to its full length, taking hold again by the front sucker and again bringing up the hindermost. The Cirripeda constitute the second class of the Articulata, which comprise the "Acorn shells" and "Barnacles" (fig. 14); these little creatures were formerly described as Molluscs, but are now considered to be articulate animals, and by some as Crustaceans. Dr. Baird gives the following description of them:--"The Cirripeds are articulated animals contained within a hard covering composed of several pieces and consisting of calcified chitine. The body of the animal is enclosed in a sac lined with the most delicate membrane of chitine, which in one group is prolonged into a peduncle and contains the ova; the body is distinctly articulated and placed with the back downwards." [Illustration: FIG. 14.--A, SEA-ACORNS; B, BARNACLES (Cirripedes).] Dr. Carpenter describes the young of the Cirripedes as not fixed like the adult creatures, but moving about freely, and only becoming fixed in the form of the usual acorn shell after undergoing several strange metamorphoses, during which stages they more resemble the ordinary Crustaceans than they do in their fixed state. The Crustacea form the third class of the Articulata, they comprise animals covered with a hard shell or carapace, like a case made up of rings, with joints allowing the pieces to move upon each other, except where several of them are consolidated to form the principal covering. These rings generally amount to twenty-one in number, some composing the claws, others the legs, &c. The shell is chiefly made of carbonate of lime (chalk) held together by animal matter. The greater part of the Crustacea inhabit the waters, and they cast their shells from time to time as they grow; the Lobster (Astacus marinus), is a familiar example, it is found in the sea near rocky shores, and is capable of rapid motion under the water, darting along (tail foremost) with great rapidity, it has powerful claws, with one of which (the blunt one) it holds its food and with the other (the sharp one) cuts it up into pieces; they have the power of casting off these claws and reproducing them; they feed upon smaller Crustacea and fish, or any animal matter they can find. The Cray-fish is another example, it is a sort of fresh-water Lobster, living in holes (frequently deserted rat holes) in the banks of rivers; its shell is very rough, but otherwise somewhat resembles the Lobster. Crabs, Shrimps, and Prawns, are members of this class (fig. 15). [Illustration: FIG. 15.--CRUSTACEA.