The next six pairs of appendages are grouped about the mouth. They are the /mandibles/, the /maxillæ/, and the /maxillipeds/. The mandibles are at the mouth-opening, and, being heavy and hard, are adapted to tearing and grinding; they have a jointed attachment, the /palpus/, whose office is to keep the mandibles clean. The two pairs of maxillæ are delicate and leaf-like. The three pairs of maxillipeds grow gradually larger, the last pair being very prominent and extending over the other mouth-parts. Next come five pairs of walking-feet. One or more pairs of these feet have pincer-like ends, or claws. Some species have the claws immensely developed, as in lobsters. The claws are the /chelæ/, and the feet which bear the /chelæ/ are termed the /chelipeds/. The rest of the walking-feet have generally single, hook-like ends, but are variously modified in different species. The abdominal segments have six pairs of appendages, also variously modified. The last segment is without appendages, but often is extended into a tail, [pg259] or fin-like expansion. The next to the last segment, in many forms, has appendages modified into swimming-plates, which extend on each side of the telson, forming a broad, fan-like caudal extremity. They have, then, to correspond to the twenty segments of the body, two pairs of sensory, six pairs of mouth-, and five pairs of walking-appendages attached to the cephalothorax, and six pairs on the abdomen. The terminal segment, or telson, is without appendages. The exopodite is present on the maxillipeds, but disappears from the walking-feet in the higher forms. In moulting the /Macrura/ split in the longitudinal line down the back; in the /Brachyura/ the split occurs across the body at the point between the thorax and the abdomen. SUBORDER ~MACRURA~ SHRIMPS, PRAWNS, LOBSTERS, CRAWFISH, AND HERMIT-CRABS The characteristic features of the /Macrura/ are an elongated body with the abdomen usually extended; a carapace, somewhat cylindrical; and the last pair of appendages of the abdomen (which are attached to the next to last segment) united with the last segment, or telson, to form a powerful caudal fin, used for swimming backward. The creeping forms in moving walk forward, but swim backward. FREE-SWIMMING FORMS: SHRIMPS AND PRAWNS