Now let us go over our Prawn--a spirit specimen--in detail. The antennae may be separated, and examined, and the appendages of the inner pair distinguished, for at first it may be thought that there are more than two pairs. This, however, is not the case, as should be ascertained by actual investigation. A needle inserted at the base of the outer antennae will separate the first three segments, bearing respectively the eyes, and the first and second pairs of antennae. The eye should be carefully looked at to make out that it is really compound. Then the joints of the antennae, each with its circle of sense-hairs, are to be noticed. Last of all, the inner pair of antennae deserve attention, for these carry in the basal joint an organ of hearing. This joint is large and sac-like, and contains an opening through which grains of sand are introduced by the animal itself. The grains serve to transmit the vibrations of the water in the sac to the auditory hairs, to each of which a branch is sent off from the auditory nerve. If the joint is opened the sand will be found. The first antennae of a lobster or crayfish may also be examined and compared. The mouth organs, of which there are six pairs, will offer some difficulty, and for this reason it may be well to pass them over in this case and to deal with these organs generally when treating of the Crab. Beneath the outer foot-jaws are the first pair of walking feet, which are used as cleansing organs. Gosse describes them as ‘beset with hairs which stand out at right angles to the length of the limb, radiating in all directions like the bristles of a bottle-brush.’ If we watch our Prawn in life, we shall frequently see these limbs in active operation. They are brought to bear on every part of the body within reach. Sowerby says[46]: ‘The prawn loves to be clean, and he takes surprising pains to keep himself so. Drawing up his tail and abdomen, he subjects their under surface to the most careful revision, scrubbing and poking between the lappets of the shell and body, diving into every crevice, and with the pincer-hand picking out every speck too large to brush away.’ The next pair of legs are also chelate; but the three following pairs are armed with claws, and it is upon the points of these that the animal walks on the bottom. The pincers of the second pair of legs are used to pick up food and bring it up to the mouth organs, where it is taken by the outer foot-jaws, and passed into the mouth. The swimming feet carry two branches, finely fringed with hairs. [Illustration: FIG. 67. First walking leg of Shrimp (enlarged).] If the carapace be removed the gills at the base of the walking feet will be exposed. These consist of thin leaf-like plates attached to a central stalk, and they are aërated by water passing in behind and out in front.