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.
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.
