368 



ZOOLOGY 



SECT. 



of the ventral valve in the Articulata. Crania (B) has the ventral 

 valve fixed directly to foreign objects, the peduncle being absent. 



The lophophore is found in its simplest form in Cistella 

 (Fig. 295, A), in which it is a horse-shoe-shaped disc with very 

 short arms, attached to the dorsal mantle-lobe and surrounded 

 with flexible tentacles which project between the valves. From 

 this the lophophore of Magellania, which may be considered as 

 typical for the Articulata, is easily derived by an increase in size, 

 and by the prolongation of the middle region of the concave edge 

 into a coiled offshoot. In the Inarticulata (C), and in Rhyn- 

 chonella (B) among the Articulata, each arm of the horse-shoe is 



FIG. 294. Typical Brachiupoda. A, Lingula ; B, Crania ; C, Discina ; D, Terebratula ; 

 E, Cistella ; F, Spirifera ; G, Kraussina. (After Bronn.) 



coiled into a conical spiral, which in some cases can be protruded 

 between the valves. 



The most noteworthy point about the muscular system is the 

 fact that the shell is both opened and closed by muscular action. 

 The dorsal valve may be taken to represent a lever of which the 

 hinge-line is the fulcrum, the cardinal process the short arm, and 

 the main portion of the valve the long arm. The muscles all arise 

 from the ventral valve, the adductors being inserted into the inner 

 face of the dorsal valve, which they depress, the divaricators into 

 the cardinal process, their action depressing it and thus elevating 

 the valve itself. In Lingula there is a ver}^ complex muscular 

 system by means of which the valves can be rubbed upon one 

 another, or moved laterally as well as opened and shut. 



