C. E. Beecher — Development of the Brachiopoda. 139 



ever, important structural differences. . An article by J. W. 

 Fewkes, " On the Larval Forms of Spirorbis borealis Daudin,'" 

 contains a nearly complete and very interesting account of the 

 development of this chsetopod. There is a striking resem- 

 blance in the characters of the cephalnla stages in both organ- 

 isms, as may be seen on comparison, figures 11, 12. Spirorbis 

 develops a posteriorly directed extension from the middle 

 segment, called a collar, which in later stages is reflexed an- 

 teriorly so as to cover more or less the cephalic portion, thus 

 agreeing with the growth and change in position of the mantle 

 in Cistella. The ventral lobe is also the larger in both. Many 

 other comparisons and homologies have been made by Morse, 19 

 and the one here described is even more marked than his 

 reference to the lobation of the cephalic collar in Sabella. Four 

 figures are introduced illustrating the principal changes in 

 Spirorbis. They may be compared with the development of 

 Cistella shown in figures 6-10. 



13 



14 



Spirorbis borealis Daudin. 



Figure 11. — Cephalula, developing lobe from the body {col.). 

 'Figure 12. — More advanced stage. 

 Figure 13. — Larval form before transformation; showing posteriorly directed 



expansion {col ) from thoracic segment. 

 Figure 14. — Transformed Spirorbis; showing folding upwards of collar partially 



enclosing head. (11-14, after Fewkes.) 



It is not intended by this to indicate a close relationship with 

 the chsetopods, for the writer is inclined to accept the opinion 

 of Joubin, 12 that the brachiopods constitute a distinct and 

 independent class. 



The Phylembryo, figure 15, differs from the Typembryo in 

 (a) the completion of the embryonic shell, or protegulum ; (b) 

 the first appearance of the tentacular lobes of the lophophore, 



