274 STUDIES IN EVOLUTION 



PART III. MORPHOLOGY OF THE BRACHIA* 



THE diagnostic value of the brachidium, or calcareous arm 

 supports, of brachiopods has long been recognized, and forms 

 one of the chief characters for generic and family sub-division 

 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 relationship 

 between the arms themselves and the brachidium, so that 

 whenever 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 arrange- 

 ment 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 well as the morpho- 

 genetic history of these organs. 



The most detailed accounts of arm development are given 

 by Brooks 5 f for Grlottidia, by Morse n for Terebratulina, and 

 by Kovalevski 10 for Cistella and Thecidea. These results, 

 combined with original observations by the writer * 2 and 

 occasional descriptions of arm structure by Davidson 7 and 

 other authors, are sufficient to include and to interpret prop- 

 erly all the leading varieties of structure. 



As shown by Brooks 5 the tentacles, or cirri, in Grlottidia 

 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. 



* Bulletin 87, U. S. Geol. Surv., Chapter IV, 105-112, 1897. 



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



