THE OLDEST NOBLE OF THEM ALL. 



747 



Fig. 2. Anatomy op a Brachiopod. 



very small body proper. In this may be found a short bowel with a 

 large liver, the nervous system, and, perhaps, also the heart. The 

 bowel has a real mouth only at the junction of the two arms, but is 

 entirely closed at the other 



end. The inner surface of 

 the valve is covered with a 

 fine, transparent skin, which 

 is called the mantle, in the 

 somewhat thickened border 

 of which are planted the stiff, 

 transparent bristles, which 

 are moved back and forth 

 by the contraction of the 

 membrane in which they are 

 fixed. The organs of gener- 

 ation also lie in the mantle, 

 and are shown in the draw- 

 ing as two lumps in the raised 

 back valve. Thus, small is 

 the body in proportion to the 

 vigorously developed arms, from which the name of the whole class, 

 brachiopods, is derived. But are they really arms ? They are hardly 

 movable. If we stick a needle into one of them, it does not stir ; and 

 there are species in which they are completely calcified. Only the 

 fringes move and respond immediately to excitations. 



We cut off a piece of the arm and place it under the microscope. 

 Each fringelet is a tube made of a firm, elastic membrane, in the hol- 

 low of which are laid one or two bundles of fibers of a muscular or 

 nervous character. On the outside the tubes are clothed with delicate 

 cells connected into a texture bearing fine, actively vibrating cilice. 

 These ciliaa generate a bubbling stream in which dance the minute 

 bodies that are floating in the sea-water. The whole stream, which 

 the cilise of the thousands and thousands of tubes produce, flows from 

 the periphery toward the mouth. The little tubes all open into the 

 chief pipe of the arm, and are, like that vessel, filled with fluid. 



With this observation a considerable number of functions are ex- 

 plained at the same time. The fluid in the little tubes and the chief 

 pipe doubtless plays a part in the movements, in that it is at times 

 pressed into the smaller vessels and expands them, and at other times 

 is held back by the foldings and contractions brought about by the 

 muscles in the chief pipe of the arm. But the constant stream which 

 the cilise keep up is all the time bringing new particles of water, heavi- 

 ily charged with oxygen, in contact with the inner surface of the man- 

 tle. An exchange of gases through their thin walls is certain to arise ; 

 the stream also produces a respiration, which is simply an exchange 

 of the carbonic acid generated within the tissues of the body for the 



