THE BRACHIOPODA 9 



the digestive tube, central nervous system and other 

 fundamental organs, occupies a surprisingly small portion 

 of the interior of the shell. From it there arise a pair of 

 muscular sheets which form a lining to the two valves, 

 and by which the valves are in fact secreted : these are 

 called the mantle-lobes, and the cavity enclosed by them 

 is called the mantle-chamber. Hanging freely in this 

 mantle-chamber are a pair of spiral ribbons, fringed with 

 tentacles along their whole length and covered with 

 microscopic vibratile hairs (cilia), the constant movement 

 of which creates the water-current which is so vitally 

 necessary, and also serve as gills (breathing organs for 

 the exchange of gases between blood and sea-water). 

 The shelly " loop " which we have seen in the fossil 

 forms serves to support these spiral " arms " (or brachia) 

 as they are misleadingly termed. In M. flavescens the 

 loop is long and doubled back on itself, the two halves 

 of this doubled-back portion being distinguished as 

 " ascending lamellae " from the longer " descending 

 lamellae " (Fig. 8). 



If we examine the interior of the valves of a modern 

 terebratuloid, we can make out certain details of structure 

 more easily than on a fossil specimen ; though after- 

 wards we shall find we can recognize the same details in 

 the fossils. The two valves are hinged together by 

 means of a pair of projections from the ventral valve 

 (hinge-teeth') which fit into two hollows (dental sockets) in a 

 horizontal shelf in front of the umbo of the dorsal valve 

 (hinge-plate). To give leverage for the opening of the 

 valves there is a short knob-like projection (cardinal 



