THE LOBSTER. 



139 



tie-brushes. These are the gill-plumes, and this room 

 is called the gill or branchial chamber. Branchial 

 comes from the Greek word meaning gills. The gills 

 are placed in two sets, six in one and twelve in the 

 other. The first row are fastened to the six feet, or 

 appendages, of the breast (thorax), which we found 

 pushing themselves up into the chamber (Fig. 116); 

 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. 

 Let us see if we can find it. There is a very curious 

 piece of machinery at the front entrance. You remem- 

 ber the oval or boat-shaped plate in front of the cham- 

 ber, formed by the hindmost 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 pro- 

 peller, 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 this 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 net- work of the blood-vessels is 

 spread over the framework of the gill-plumes, just as 



