The eye of a crustacean is a very complicated structure, commonly described as a compound eye. It consists of a large number of conical, radiating, crystalline rods, collected together into a mass that presents a convex outer surface. This surface is covered with a transparent layer of chitin which naturally presents a more or less distinct netted appearance, the bases of the rods being in contact with its inner surface, and visible through it. Each rod is surrounded by a layer of pigment that prevents light from passing from one to another, and the optic nerve passing into the base of the compound structure sends a sensitive filament into each one. [Illustration: FIG. 189.--SECTION THROUGH THE COMPOUND EYE OF AN ARTHROPOD] In many crustaceans this compound eye is situated on the end of a movable stalk that generally allows it to be protruded or drawn under cover as occasion requires, but in others the organ does not project beyond the general surface of the body. Thus we hear of the animals of this class being divided into the _stalk-eyed_ and the _sessile-eyed_ groups; the former being represented by crabs, lobsters, shrimps, &c.; and the latter by sandhoppers and sandborers. Crustaceans undergo metamorphoses while very young, the body being altered considerably in form at several successive moults. Some, in their earliest stage, consist of a little oval body that shows no signs of a division into segments. It swims about by means of three pairs of appendages, and has only one eye. Others start life with four pairs of limbs, attached to the front portion of the body, a segmented abdomen, as yet perfectly limbless, and a pair of compound eyes. Then as the successive moultings take place, new segments and new appendages are developed, until, at last, the form of the adult is assumed. The accompanying illustration shows four stages in the development of the Common Shore Crab. [Illustration: FIG. 190.--FOUR STAGES IN THE DEVELOPMENT OF THE COMMON SHORE CRAB] The lowest division of the crustaceans contains the _Cirripedia_ or Curl-footed crustaceans, which includes the Barnacles that are so frequently seen attached to the bottom of ships and of floating timber, and the Acorn Barnacles, the conical shells of which often completely cover large masses of rock on our shores. [Illustration: FIG. 191. THE BARNACLE]