BRACHIOPODA. 



577 



FIG. 460. Diagrams showing course of buccal groove in a, 

 Argiope; b, WaJdheimia and c, Rhynchonella. m mouth; 

 Hip ; t tentacles. 



proximal part, extending almost as far as the mouth. This may be 



called the proximal loop of the buccal groove; its two limbs are 



united by a mem- 

 brane. The groove 



now turns ventral- 

 wards, and again 



passes forwards into 



a vertically placed 



spiral, which is coiled 



on to the dorsal valve 



and at the apex of 



which it terminates. 



The spiral part is 



united with the spiral 



part of the opposite 



side by a membrane. 



The course of the 



buccal groove in 



Waldheimia is shown 



in the diagram (Fig. 



460). The proximal 



loop of the apparatus is supported in Waldheimia and Terebratula 



by a calcareous process of the dorsal valve (Fig. 461). In Rhyn- 

 chonella there is no calcareous 

 support for the arms, but the 

 buccal groove is prolonged into a 

 long, horizontally placed spiral on 

 each side, the apex of the spiral 

 being directed to the dorsal side 

 (Figs. 460 c, 463). In Lingula the 

 arms are very much as they are 

 in Rhynchonella, the spiral being 

 horizontally placed with its apex 

 towards the dorsal valve. 



In Lingula and Rhynchonella it 

 appears that the animal has the 

 power of protruding its arms from 

 the shell. The epithelium of the 

 buccal groove and the tentacles is 



ciliated, and the whole apparatus is to be regarded as a food- 

 procuring organ. 



2 p 



FIG. 461. Dorsal valve of the shell of 

 Waldheimia atistralis, with the brachial 

 skeleton (after Hancock). 



