BRACHIOPODA 615 



layer of organic material (pertostracum)^ under which is a thin layer 

 of pure calcium carbonate and a thick inner prismatic layer composed 

 mainly of calcareous but partly of organic material. The shell valves 

 are opened and closed by a muscle system which is much more 

 complicated than that of the lamellibranchs. 



The hinge line is posterior and the mantle cavity is thus anterior. 

 On opening the shells it is seen to be largely occupied by a compli- 

 cated organ known as the lophophore of which a description follows. 

 The mouth is placed in a transverse groove which is bounded, dorsally 

 by a continuous lip and ventrally by a row of tentacles. The groove 

 is enormously extended and its boundaries drawn out laterally into 

 two arms which are often coiled spirally in these and other members 

 of the phylum. The tentacles are long and may be protruded from 

 the shell opening. The cilia on the tentacles and on the mantle sur- 



Fig. 425- 



-jiL.ped. 



^■;- 



Fig. 425. Crania attached to a stone in the act of feeding with protruded 

 tentacles. AA, ingoing, B, outgoing currents. After Orton. 



Fig. 426. Lingula in positions of life in mud (indicated by stippling), i, feed- 

 ing position with peduncle {ped.) extended; 2, position when peduncle is 

 contracted ; ch. chaetae fringing entrance to shell. Arrows indicate currents. 



faces produce two ingoing currents of water at the sides opposite the 

 two arms of the lophophore; the outgoing current is central, between 

 the two arms (Fig. 425). This ciliary mechanism is similar to that of 

 the lamellibranch ctenidium. On each side the current of water is 



