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THE POPULAR SCIENCE MONTHLY 



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 cu- 

 rious 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 mechan- 

 ism of the screw was all worked out in our little lobster long years 

 before it was discovered by the great Archimedes. The tiny net-work 

 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 lob- 

 ster's breathing or respiration. We will leave his circulation, his 

 muscles and nerves, for another chapter. 



Window;. 



Eye. 



Eye-Stalk. 



Outer Cover, or 

 Cornea, of Eye. 



Eod. 



Cone 

 Coloring Matter. 



Nerve of Sight. 



Fio. 26. 



Fig. 27. The Lobster's Eye, showing 

 now the Nerve is joined to TnE Win- 

 dows IN FRONT BY THE KODS AND CONES. 



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 lob- 



