CHAPTER IV. 



MORPHOIiOGY OF THE BRACHIA. 



By Charles E. Beecher.i 



The diagnostic value of the brachidiiim, or calcareous arm sujtports, 

 of brachiopods has long been recognized, and forms one of the chief 

 characters for generic and family subdivision among the Terebratulacea 

 and Spiriferacea. This character fails in all other brachiopods, which 

 have simply fleshy arms, unsupported by calcareous skeletons. There 

 is, however, generally the most obvious analogy and intimate relation- 

 ship between the arms themselves and the brachidium, so that when- 

 ever either structure can be ascertained it furnishes important data 

 aiding in the determination of the systematic position of any genus 

 within a family or order. 



The growth of the arms, or lophophore, in recent genera may be 

 divided into distinct stages, which often have a direct correlation with 

 other important features of the shell. In many cases it is also possible 

 to infer the form and arrangement of the brachia in fossil genera from 

 markings on the interior of the valves and from the calcareous arm 

 supports, and thus to obtain the chronogenetic as weU as the morpho- 

 genetic history of these organs. 



The most detailed accounts of arm development are given by Brooks^ 

 for Glottidia, l)y Morse" for Terebratulina, and by Kovalevski^" for 

 Cistella and Thecidea. These results, combined with original observa- 

 tions by the Avriter'^ and occasional descriptions of arm structure by 

 Davidson^ and other authors, are sufficient to include and properly 

 interpret all the leading varieties of structure. 



As shown by Brooks,-^ the tentacles, or cirri, in Glottidia originate 

 on the dorsal side of the oral disk. They grow in pairs, one on each 

 side of a central lobe. New tentacles are added between the first pair 

 formed and the median lobe. Thus the cirri farthest removed from 

 the median lobe are the oldest. Tentacles are added rapidly until the 

 first arc is extended to a semicircle, and then progressively the whole 

 disk becomes surrounded by a circle of these organs. The further 

 introduction of cirri can only take place by the enlargement of the 

 oral disk or through the deformation of the circle by lobes, loops, or 

 extensions. In Glottidia, Lingula, Discinisca, Crania, and Ilhyiicho- 

 nella the two points of tentacular increase, originally together aud on 



iThe references to the literature will be found at the end of this chapter. 



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