PHYLUM MOLLUSCO1DA 



339 



in Magellania, sometimes attached to the septum or to the interior 

 of the dorsal valve (E), sometimes, as in the extinct Spirifcra, 

 represented by a complex double spiral (F), sometimes reduced to 

 short paired rods springing from the septum (G). 



The majority of both orders are attached by a longer or shorter 



peduncle which passes between the proximal ends of the valves in 



Lingula (Fig. 270, A), through a perforation in the ventral valve in 



>iscina (C), and through a foramen in the spout-like posterior end 



FIG. 270. Typical Brachiopoda, A, Lingula; B, Crania; C, Discina j D, Terebratula; 

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



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. 271, 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 vajves. 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 

 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. 



z 2 



