BRACHIOPODA 



539 



The hinge hne 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. The mouth is placed in a trans- 

 verse 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 ten- 

 tacles are long and may be protruded from the shell opening. The 

 cilia on the tentacles and on the mantle surfaces produce two ingoing 



[^■-ped. 



Fig. 400. 

 tentacles. 



Fig. 401. 



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

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



Fig. 401. Lingula in positions of life in mud (indicated by stippling). 

 I, feeding position with peduncle (ped.) extended; 2, position when peduncle 

 is contracted; ch. chaetae fringing entrance to shell. Arrows indicate cur- 

 rents. 



currents of water at the sides opposite the two arms of the lophophore ; 

 the outgoing current is central, between the two arms (Fig. 400). This 

 ciliary mechanism is similar to that of the lamellibranch ctenidium. On 

 each side the current of water is directed between the tentacles of the 

 lophophore, and the smaller and lighter particles suspended in it are 

 sieved away and pass into the ciliated buccal groove and so towards 

 the mouth. Heavier particles drop on to the ventral mantle lobe and 

 are removed by outgoing ciliary currents and sudden clapping move- 

 ments of the valves. When the ingoing currents have passed between 



