Examine the appendages of the cephalothorax. Like the appendages of the abdomen the typical composition of each includes a protopodite, an exopodite and an endopodite, but some of these appendages are much modified, and show a loss of one of these parts, or the addition of an extra part. The cephalothoracic appendages may be divided into three groups, an anterior group of three pairs of mouth-parts (belonging to the head) of which the first pair is the _mandibles_ and the others are the _maxillæ_; a second group of three pairs of foot-jaws or _maxillipeds_, belonging to the thorax, and a third group of five pairs of _walking-legs_. The mandibles, lying next to the mouth-opening, are hard and jaw-like and lack the exopodite; the first maxillæ are small and also lack the exopodite; the second maxillæ have a large paddle-like structure which extends back over the gills on each side within the space, the _branchial chamber_, above the gills. It is by means of this paddle-like structure (the _scaphognathite_) that currents of water are kept up through the gill-chambers. The maxillipeds increase in size from first to third pair. Each pair of walking-legs except the last bears _gills_. These gills are the organs by which the blood is purified. The blood of the crayfish flows into the large vessels on the outer sides of the gill and thence into the fine vessels in the little leaf-like lamellæ. At the same time the air which is mixed with the water bathing the gills passes freely through the thin membranous walls of these lamellæ and blood-vessels, and the blood gives off its carbonic acid gas to the water and takes up oxygen from the air in the water. Thus it will be seen that the office of the gill is like that of the lung in the toad, namely, to act as an organ for the elimination of carbonic acid gas and the taking up of oxygen. Note the pincer-like appendages of the first pair of legs. These pincers are the _chelæ_, with which food is torn into bits and placed in the mouth. In the basal segment of each of the last pair of legs of the male note the _genital pore_. In the female the _genital pores_ are in the basal segments of the next to last pair of legs. Is the crayfish bilaterally symmetrical? Note the repetition of parts in the crayfish, that is, the recurrence of similar parts in successive segments. This serial repetition of parts among animals is called _metemerism_. =Internal structure= (fig. 4).--TECHNICAL NOTE.--With a pair of scissors cut through the dorsal wall of the cephalothorax into the body-cavity. Cut the body-wall away from both sides and remove the middle portion. [Illustration: FIG. 4.--Diagrammatic median longitudinal section of crayfish (_Cambarus_ sp.).]