DEVELOl'MEXT OF THE BRACHIOPODA 



253 



parisons, which may be consulted elsewhere,*' ^^^ ^^' ^^-^^- 21 

 attention is called to the similarity of development of the 

 hrachiopod typembryo to the larval stages of Spirorbis. 

 There are, however, important structural differences. An 

 article by J. W. Fewkes, "On the Larval Forms of tSpirorlis 

 borealis Daudin," " contains a nearly complete and very inter- 

 esting account of the development of this chtetopod. There 

 is a striking resemblance in the characters of the cephalula 



97 



98 



96 



Spirorbis borealis Daudin. 



FlGUHE 95. — Cephalula, developing lobe from the body (col). 



FiGUKE 96. — More advanced stage. 



PiiiURE 97. — Larval form before transformation ; showing posteriorly directed 

 expansion {col] from thoracic segment. 



FiGDRE 98. — Transformed Spirorbis ; showing folding upward of collar 

 partially enclosing head. (Figures 95-98, after Fewkes.) 



stages in both organisms, as may be seen on comparison 

 (figures 95 and 96). Spirorbis develops a posteriorly directed 

 extension from the middle segment, called a collar, which in 

 later stages is reflexed anteriorly 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,^^ and the one here 

 described is even more marked than his reference to the 



