Page:Popular Science Monthly Volume 6.djvu/604

586 the six feet or appendages of the breast (thorax) which we found pushing themselves up into the chamber (Fig. 22). The other twelve are fastened to the pleuron or side-pieces of the cephalo-thorax. These gills are not covered with stiff hairs (cilia) as the mussel's, so there must be some other plan of moving the water. There is a very curious piece of machinery at the front entrance. You remember the oval or boat-shaped plate in front of the chamber, formed by the hind-most little jaw (maxilla). This plate is called the scapho-gnathite, which means the little skiff-like jaw. It is made on the plan of the Archimedean screw, and it works as the screw of a propeller, and is set in motion by the jaws. The water enters the back part of the gill-chamber by a slit, and it is scooped out by the screw through the opening in front, bubbling and frothing as it goes. Thus the mechanism of the screw was all worked out in our little lobster long years before it was discovered by the great Archimedes. The tiny network of the blood-vessels is spread over the framework of the gill-plumes, just as you found it on the lattice-work of the mussel's gill-pockets. As the screw propels the water through the branchial or gill chamber the blood takes out the oxygen from the air in the water, and gives back carbonic acid. You remember how the strong hairs (cilia) of the pockets sweep the water along over the mussel's gills, and how the little blood-vessels take up their oxygen and give up their carbonic acid. The gills that are fastened to the legs move when the legs move, and the faster they go the more water they use. So much for the lobster's breathing or respiration. We will leave his circulation, his muscles and nerves, for another chapter.

The eyes, as you have seen, are away in front (Figs. 15, 22) at the ends of the first pair of appendages—the eye-stalks. The eye is kidney-shaped; instead of having one window or pupil as your eye has, through which the light enters, the whole front is divided into squares like old-fashioned window-panes (Fig. 26). Each square is really a separate eye, and this is what is called a compound eye. The