208 SYNOPTIC COLLECTION. 



stage when the shell has assumed the more rounded out- 

 line of Obolella. That this stage is more specialized than 

 the Paterina-stage is proved by the fact, that, whenever 

 the two stages occur in the development of a species, the 

 Obolella-stage always follows the Paterina-stage. The 

 hinge line grows narrower in the neanic (fig. 2) and the 

 ephebic stage (fig. 3), while the shell becomes tongue- 

 shaped in form. 



The larva of the living Lingula is free swimming, but 

 soon a pedicle is developed which becomes a long flexible 

 organ in the adult (No. 654). This pedicle is not 

 attached, but the end is buried in the sand and protected 

 by a tube made of sand grains, after the fashion of the 

 protective coverings of worms. According to Morse 1 the 

 Lingula partially recedes into its sand tube after the 

 manner of worms. The shell is in a line with the pedicle, 

 and the length and flexibility of the latter organ allow the 

 shell freedom of motion in any direction. 



Beecher 2 has shown that physical forces acting simi- 

 larly on all sides of a shell, which in this case is made 

 possible by the long pedicle, tend to produce equal valves, 

 such as are seen in Lingula. 



Comparative simplicity of structure marks the internal 

 organs as well as the external characters of the Atremata. 

 The fleshy body takes up the greater part of the shell, 

 while the arms or brachia (the characteristic organs which 

 have given the name of Brachiopoda or arm-footed animals 

 to the group) are without limy supports of any kind. 

 Since these organs are of importance in determining the 

 phylogenetic relations of genera, we will give figures to 

 illustrate their stages of development. Their mode of 

 growth "is alike in the larval stages of all Brachiopods." 

 They first develop tentacles in pairs on each side of the 

 median line in front of the mouth. This stage is repre- 



1 Proc. Boston Soc. Nat. Hist., XV, 1873, p. 372. 



2 Amer. Journ. Sci., (3), XLI, April, 1891. 



