We shall not be able at present to repeat Hensen’s experiment, but we may break up the sac and extract the otolith, which may be seen with the lenses at our command.
Small specimens of the Shore Crab (Car´cinus mae´nas) are fair game for us. They will interest us while living in the aquarium, and when dead we can put them into pickle, and break them up at our leisure.
The broad shell of the Crab—the crab-cart of children—corresponds to the carapace of the Lobster, the Prawn, and the Shrimp, and bears the same number of appendages—fourteen pairs. To make out the pleon or swimming part, it is only necessary to lay the crab on its back, and, with a needle, or small knife, turn back the flap—or ‘apron,’ as fishermen call it—which lies in a groove on the under surface. Here we shall find the pleopods, or swimming feet, though they are not really used for that purpose. The eyes, the two pairs of antennae, and the five pairs of walking legs will offer no difficulty. It is only necessary to remark that the terminal joints of the last pair of walking legs are flattened and fringed with hair, showing some approach to the swimming crabs, which use those organs to swim with.
Now we may examine the mouth organs, of which there are six pairs. To do this, the crab may be fixed, with the back downwards, or held lightly but firmly in the left hand. The latter plan is perhaps the more convenient. The index and middle fingers should support the carapace, and the thumb should be placed on the pleon. The outer pair of mouth organs are the third maxillipedes, or jaw-feet. These close the area of the mouth, somewhat after the fashion of the double-doors of a cupboard, though the hinging, of course, is different. To open these jaw-feet, a needle should be inserted at the top, with a gentle pressure downwards and outwards. The back of the crab is turned away from us, so that the left jaw-foot should be pressed outwards to the right, and the right jaw-foot to the left.
Theoretically these limbs consist of the same number of joints as the perei´opods or walking legs; and this is to be borne in mind, even if we do not succeed—and we probably shall not—in tracing the full number of seven joints. But we may notice and count the terminal joints, and observe the fringing of the limb with hair.
A similar method of using the needle will enable us to raise the second and first pairs of maxillipedes, which are of smaller size and softer structure.
Having raised these organs, it is well to replace them—to close the doors, as it were—and then to raise them again, to observe how they work. They may then be detached and fastened to a small piece of card, for comparison with similar organs in the lobster and the crayfish, and with the mouth organs of insects.
Beneath the maxillipedes are the second and first maxillae—thin, leaf-like organs. The first-named are furnished with spoon-like scoops, which serve to carry out from the gill-chamber the water that has parted with its oxygen in aërating the gills.
Immediately below the maxillae lie the mandibles, with hard, cutting edges, by means of which the food is broken up. Each carries a palp.
