Attached just in front of the legs the crayfish has three pairs of finger-like appendages, called foot jaws (maxillipeds), with which it passes the food from the large pincers to its mouth (Figs. 85, 86). They are in form and in use more like fingers than feet. In front of the foot jaws are two pairs of thin jaws (maxillæ) and in front of the thin jaws are a pair of stout jaws (mandibles) (Fig. 85). Do the jaws move sidewise or up and down? Which of the jaws has a jointed finger (palp) attached to it? Do all the appendages for taking food have both exopod and endopod branches on a basal stalk or protopod? Which of the appendages have a scalloped edge? How would you know from looking at the crayfish that it is not merely a scavenger? Why are there no pincers on the hind feet? [Illustration: FIG. 87.—Gill cover removed and gills exposed. _Mp_, gill bailer. ] =Sense Organs.=—Find the _antennæ_, or long feelers (Figs. 82, 90). Are the antennæ attached above or below the eyes? (Fig. 87.) [Illustration: FIG. 88.—LENGTHWISE SECTION OF MALE CRAWFISH. _c_, heart; _Ac_, artery to head; _Aa_, artery to abdomen; _Km_, stomach; _D_, intestine; _L_, liver; _T_, spermary; _Go_, opening of sperm duct; _G_, brain; _N_, nerve chain. ] Find the pair of _antennules_, or small feelers. Are their divisions like or unlike each other? Compare the length of the antennules and the antennæ. Compare the flexibility of the antennæ with that of the other appendages. Observe the position of the _eyes_ (Figs. 81, 88). How long are the eyestalks? Is the stalk flexible or stiff? Touch the eye. Where is the joint which enables the stalk to move? Is the outer covering of the eye hard or soft? A mounted preparation of the transparent covering (cornea) of the eye, seen with lower power of microscope, reveals that the cornea is made up of many divisions, called facets. Each facet is the front of a very small eye, hundreds of which make up the whole eye, which is therefore called a compound eye. The elongated openings to the _ear sacs_ are located each on the upper side of the base of a small feeler just below the eye. =Respiratory System.=—The respiratory organs are gills located on each side of the thorax in a space between the carapace and the body (Fig. 87). The gills are white, curved, and feathery. Is the front gill the largest or the smallest? The gills overlap each other; which is the outermost gill? On the second maxilla is a thin, doubly curved plate called a gill bailer (Fig. 85). The second maxilla is so placed that the gill bailer comes at the front end of the gill chamber. The bailer paddles continually, bringing the water forward out of the gill. The gills are attached below at the base of the legs. Are the gills thick or thin? How far upward do they go? Does the backward motion in swimming aid or hinder the passage of the water through the gills? Does a crayfish, when at rest on the bottom of a stream, have its head up or down stream? Why?