DR. T. DAVIDSON ON RECENT BRACHIOPODA. 



47 



genus, a pair of very remarkable organs, eminently cbaraetcristie of the Brachiopoda. 

 'J'hey are often and more correctly termed labial appendages, on account of each member 

 being a prolongation of the lateral portions of the lips of the margins of the mouth. 

 They occvipy the larger portion of the cavity of the shell in front of the visceral chamber ; 



FiK. 0. 



Fis;. 7. 



Wahlheimia Jlavescen.t. 



Vi". 6. Interior of dorsal valve, to show the position of the labial appendages. (A portion of the fringe of the 

 cirri has been removed to show the brachial membrane and a portion of the spiral extremities of the 

 arms.) Enlarged. 



Fig. 7. Longitudinal section, with a portion of the animal. (/, 7;, brachial appendages ; a, adductor : c, c', divari- 

 cator muscles ; s, septum ; v, mouth ; :, extremity of alimeutarj* tube. (The peduncular muscles have 

 been purposelj- omitted.) Enlarged. 



and are mainly composed of a membranous tube fringed on one side Avith long flexible 

 hollow cirri, which are not capal)le of being protruded in those families and genera in 

 which they ai"e folded back upon themselves and supported T)y a skeleton, as in JFald- 

 heiiJiia. Hancock says (Phil. Trans, vol. cxlviii. p. cxliii.) that the arms or labial appendages 

 " are normally composed of a membranous tube or canal bearing a semicartilaginous 

 grooved ridge. The latter stretches from end to end of the former, and gives support 

 to the fringe of cirri. As far back as the commencement of the spirals the arms are as 

 above stated ; but for the entire length of the lateral portions of the loop, wiiere the 

 arms are doubled ujion themselves, and Avhere, of course, two tvibes or canals might have 

 been expected, thex'e is only one, the two having, as it were, coalesced. These large canals 

 at the roots of the arms are continuous with those of the spirals, and terminate in blind 

 sacs, one at each side of the oesophagus, close to the mouth. On making a transverse 

 section of this part of the arm, the enlarged terminal portion of the brachial corn' i- 

 seen connected with the external edges of the dorsal and ventral mem])ers of the loop ; 

 and the pallial lobule, stretching between the inner edges, forms a sort of inner tube. 

 This inner tul^e opens widely into the perivisceral chambei*, is in fact a prolongation of 



