called swimmerets or little swimmers. The lower joints of these paddles have two broad, flat toes. The paddles on the last or sixth somite are different from the others; they are wider and turned backward (Fig. 15) so as to lie at each side of the tail-piece, telson; and these great-fingered paddles, taken with the telson, form what is called the tail-fin. The under or ventral part of each somite, which lies between the paddles, is called the sternum. The rounded upper or dorsal part of the body-piece is the tergum, which means the back. In front of the abdomen, with its somites, is the cephalo-thorax. This cephalo-thorax has a tergum, or back part, a sternum, or under part, a pleuron, or side part (Fig. 15), and so many things
Fig. 16.—One of Lobster's Body-pieces, or Somites.
are hanging down from it one can hardly count, much less learn them. Counting from behind forward, you will find between the lobster's body, or abdomen, and the head, eight pair of jointed legs, one pair much longer and larger than the others, with huge pincers at the ends. All these eight pair are called the thoracic appendages, because they are fastened to the thorax, or breastplate. The lobster uses the four back-pairs for walking, and so they are called the ambulatory limbs. The last pair has seven joints, and every joint works in a different direction; so, when these hind-legs start off, it is hard to tell where they intend to go. The next pair of walking-legs are like the hindmost pair, except that the first joint sends out a piece above it, which is kept out of sight in a little room in the side of the lobster (Fig. 22). We shall say more about this room by-and-by. The two front pair of walking-legs send up pieces also into this chamber, but the end of the leg is different from the last two pairs, for they
