282 



STUDIES IN EVOLUTION 



line of the cirrated margin of the lophophore, exclusive of 

 the arm. Therefore it is impossible in these and closely 

 allied genera to infer the stage of development of the lopho- 

 phore from the loop alone. Dyscolia is an excellent example, 

 124 _ since the loop is the same as in 



Terebratulina ; but the lophophores 

 are quite distinct in each, the 

 former being of the trocholophus 

 type and the latter belonging to the 

 Trocholophus. plectolophus. 



Taxolophus. 



Spirolophus Stage. 



The last type to be noticed is 

 the one in which there are two 



Schizolophus. -T -I i 



separate coiled arms, each with a 

 row of cirri on one edge only (fig- 

 ure 124, d, e). It embraces the 

 greater part of the families of brach- 

 Spirolophus. io pods in the orders Telotremata 

 and Protremata, and includes all 

 the living species in the orders 

 Atremata and Neotremata. 



In the early stages of develop- 

 ment of the spiral lophophore there 

 is an agreement with the early 

 FIGURE 124. Early stages stages of the families alreadv no- 



of lophophore of Glottidia, and , i -, ., , , , 



adult brachia in Lingula and tlCed ' and the taxolophus, trochol- 

 Hemithyris. a, b, c, early stages Ophus, and Schizolophus Stages may 



Brooks.) d, adult brachia in The separation and growth of the 



Lingula. (After Woodward.) sp i ra l arms seem to be due to the 

 e, adult brachia in Hemithyris . -. . 



psittacea. (After Hancock.) widening or expansion of the me- 

 dian lobe or tentacle, on each side 



of which is the formative tissue for new cirri. This is very 

 apparent in the young Ducinisca described by Muller, 12 and 

 the G-lottidia described by Brooks. 6 



The brachidium in Zygospira passes through a series of 



