SCIIUCHKRT. 



SPIROT.OPHUS STAGE. 



Ill 



ceutrouelliform stage. Since approximately this form of bracliidium 

 is also {'hai'a(;teristi('. of the young' of decent terebrahiloids, it maybe 

 taken in Zygospiia as indicative of the trocholophus stage of brachial 

 development. \\'ith this as a starting point for comparison, the further 

 correlation of the succeeding stages is very simple. 



The first resorption of the end of the loop in Zygospira produced a 

 schizolophus condition, and fuither resorption carried the brachidium 

 to a stage closely resembling ])ielasina (fig. G/>). The dielasmatiform 

 stage has already been explained as due to the requirements of space 

 for the growth of the coiled biachia. Next, the initial calcification of 

 the spiral arms resulted in the extension of the descending branches 

 beyond the Juguni (fig. Oc), and, lastly, complete calcification manifests 

 the spirolophus structure and i)roduced the characteristic brachidium 

 of the Spiriferacea. 



The Atrypida? and the Athyrida' seem to stand to each other in the 

 same relation as the Terebratel- 

 lidie and Terebratulida\ In tlie 

 flrstthe descending branches are 

 widely separated and follow the 

 edges of the valves ; in the sec- 

 ond the descending branches are 

 close together. This difference 

 in the Spiriferacea produces the 

 converging cones of the Atryp- 

 idaf! (fig. Gd) and the diverging 

 cones of the Athyrida-, Si^irifer- 

 ida*, Eetziida^ (fig. Ge), etc. 



It seems doubtful whether the 

 flesh}'- j)ortionsof the brachia in 

 the Meristellidu' and Athyrida' 

 possessed additional characters 

 expressing the complexity and elaboration reached by the jugal proc- 

 esses, even when the lamella; were duplicated, as in Koninckina and 

 Kayseria. 



From the above descriptions and illustrations it appears that the 

 mode of growth of the cirrated lophophore, or brachia, is alike in the 

 larval stages of all brachiopods. They first develop tentacles in jiairs 

 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 introverted in front (schizo- 

 lophus stage). From this common and simple structure all the higher 

 types of brachial complication are developed through one of two 

 methods: (1) The growing 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 



EiG. 6. — Metamorphoses of brachidium of Zygo- 

 spira and adult brachidium of Rhyncliospira. a, b, c, 

 (/, metamorphoses of bracliidium of Zygospira recur- 

 virostra, enl. (after Beecher and Schuchert). e, Bra- 

 chidium of Iihi/nchospira evax (after Beecher and 

 Clarke). 



