Astacus, the Crayfish, a useful type of higher Crustacea. It lives in streams, walking on its long limbs or swimming backwards by the action of its tail-like abdomen. The body is composed of twenty segments, which are very dissimilar in appearance, though constructed on the same type. The anterior thirteen segments are fused to form the strong "cephalothorax," covered by a shield or carapace; the remaining seven form the abdomen. Each segment, except the last, has a pair of appendages, very variable in form, but all constructed on the same fundamental plan. The six posterior pairs are swimmerets. These are preceded by eight pairs on the thorax, of which the posterior five are long walking limbs (three are clawed), and the three anterior are known as maxillipedes or jaw-feet; the appendages on the head consist of three pairs of jaws and two of feelers (antenna and antennule). There are a considerable number of gills on each side. The heart is dorsal and in a large space known as the pericardium. The mouth leads by a short oesophagus to a stomach armed with a complex masticatory apparatus. The sexes are separate. The eyes are on short stalks, one on each side of the head. It undergoes ecdysis, i.e. the skeleton is periodically thrown off to admit of growth.