[Illustration: FIG. 3.--Ventral aspect of crayfish (_Cambarus_ sp.), with the appendages of one side disarticulated.] At the anterior end of the cephalothorax note a sharp projection, the _rostrum_. Where are the eyes? Remove one of them and examine its outer surface with a microscope. A bit of the outer wall should be torn off and mounted on a glass slide. Note that it is made up of a great many little facets placed side by side. Each of these facets is the external window of an eye element or _ommatidium_. An eye composed in this way is called a _compound eye_. In front of the eyes note two pairs of slender many-segmented appendages. The shorter pair, the _antennules_, are two-branched. Remove one of them and note at its base a small slit along the upper surface. This slit opens into a small bag-like structure which contains fine sand-grains. The bag is protected by a series of fine bristles along the edge of the slit. This bag-like structure is believed to be an auditory organ. The longer pair of appendages are the _antennæ_, and in the fine hair-like projections upon the joints is believed to be located the sense of smell. Thus it will be seen that the sense-organs of the crayfish, like those of the toad, are located on the head. Beneath the basal portion of each antenna there is a flat plate-like projection, at the base of which on the upper edge will be noted a small opening, the exit of the kidney, or _green gland_. Make a drawing of the surface of part of an eye; also of an antennule; and of an antenna. TECHNICAL NOTE.--Stick one point of the scissors under the posterior end of the carapace on the right side, and cut forward, thus exposing a large cavity, the gill-chamber. Remove all of the mouth-parts, legs and abdominal appendages from the right side, being careful to leave the fringe-like parts, the gills, attached to their respective legs. Place all of the appendages in order on a piece of cardboard. Examine the abdominal appendages, called _pleopods_, or swimming feet. How many pairs are there? Each is composed of a basal part, the _protopodite_, and two terminal segments, an inner one, the _endopodite_, and an outer, the _exopodite_. In the males the first and second pleopods of the abdomen are larger and less flexible than the others. In the female the pleopods serve to carry the eggs and the first two pairs are very small or absent. Note the last set of abdominal appendages. These are the _uropods_, which together with the telson form the tail. Make a drawing of the pleopods of one side.