282 



STUDIES IN EVOLUTION 



124 



Taxolophus. 



Schizolophus. 



line of the cirnited 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, 



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. 



Spirolophus Stage. 



The Last type to be noticed is 

 the one in which there are two 

 separate coiled arms, each with a 

 row of cirri on one edge only (fig- 

 ure 124, f?, e). It embraces the 

 greater part of the families of brach- 

 iopods 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 

 stages of the families already no- 

 ticed, and the taxolophus, trochol- 

 ophus, and schizolophus stages may 

 be determined (figure 124, a, h, c). 

 The separation and growth of the 

 spiral arms seem to be due to the 

 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 Discinisca described by Midler, ^^ j^^j 

 the Glottidia described by Brooks.^ 



The brachidium in Zygospira passes through a series of 



Spirolophus. 



Figure 124. — Early stages 

 of lophophore of Glottidia, and 

 adult brachia in Lingnla and 

 Heviiihi/ris. a, h, c, early stages 

 of lophophore of Glottidia py- 

 ramiddta. Enlarged. (After 

 Brooks.) d, adult brachia in 

 Lingula. (After Woodward.) 

 e, adult brachia in Hi'mithi/ris 

 psiliaren. (After Hancock.) 



