284 STUDIES IN EVOLUTION 



brachia, 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 (taxolophus stage). New 

 tentacles are continually added at the same points, until, by 

 pushing back the older ones, they form a complete circle 

 about the mouth (trocholophus stage), later becoming intro- 

 verted in front (schizolophus stage). From this common and 

 simple structure all the higher types of brachial complication 

 are developed through one of two methods : (1) The growing 



125 



FIGURE 125. Metamorphoses of brachidium of Zygospira and adult bra- 

 chidium of Rhynchospira. a, b, c, d, metamorphoses of brachidium of Zygospira 

 recitrvirostris. Enlarged. (After Beecher and Schuchert.) e, brachidium of 

 Homceospira evax. (After Beecher and Clarke.) 



points of the lophophore, or points at which new tentacles 

 are formed, remain in juxtaposition; or (2) they separate. 

 Complexity in the first is produced (a) by lobation, as in 

 Megathyris, Eudesella, Pterophloios Thecidea, etc. (ptycholo- 

 phus type), and (b) by looping (zugolophus) and the growth 

 of a median, unpaired coiled arm (plectolophus), as in Magel- 

 lania, Terebratulina, etc. ; in the second (c) by the growth of 

 two, separate, coiled extensions or arms, one on each side 

 of the median line (spirolophus), as in Lingula, Crania, Dis- 

 cinisca, Rhynchonella, Leptcena, Davidsonia, Spirifer, Athyris, 

 Atrypa, etc. 



