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 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 crawfish, when at rest on the bottom of a stream, have its head up or down stream? Why?
Openings.—The slitlike vent is on the under side of the telson (Figs. 82, 88). The mouth is on the under side of the thorax behind the mandibles. At the base of the long antennæ are the openings from the green glands, two glands in the head which serve as kidneys (Fig. 89). The openings of the reproductive organs are on the third
