=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 pair of legs in the female, and the fifth pair of legs in the male (Fig. 88). The eggs are carried on the swimmerets. [Illustration: FIG. 89.--Level lengthwise section showing _h_, heart. _d_, green gland. _le_, liver. _kie_, gills. _kh_, gill cavity. _ma_, stomach. (After Huxley.)] =Internal Structure.=--SUGGESTIONS. If studied by dissection, it will be necessary to have several crawfish for each pupil, one for gaining general knowledge, and others for studying the systems in detail. Specimens should have lain in alcohol for several days. [Illustration: FIG. 90.--SECTION OF CRAWFISH showing stomach _s_, liver _li_, and vent _a_.] =The Food Tube.=--Is the stomach in the head portion of the cephalothorax or in the thoracic portion? (Figs. 88, 89). Is the stomach large or small? What is its general shape? Does the gullet lead upward or backward? Is it long or short? (Fig. 88.) The mid tube, which is the next portion of the food tube, is smaller than the stomach. On each side of it are openings from the bile ducts which bring the secretion from the digestive gland, sometimes called the liver. Does this gland extend the whole length of the thorax? Is it near the floor or the top of the cavity? The third and last portion of the food tube is the intestine. It extends from the thorax to the vent. Is it large or small? Straight or curved? The powerful flexor muscles of the tail lie in the abdomen below the intestines. Compare the size of these muscles with the extensor muscle above the intestine (Fig. 90). Why this difference? Does the food tube extend into the telson? Locate the vent (Fig. 90).