1902.] CONKLIN — EMBRYOLOGY OF A BRACHIOPOD. 41 



THE EMBRYOLOGY OF A BRACHIOPOD, 

 TEREBRATULINA SEPTENTRIONALIS Couthouy. 



EDWIN G. COHKLIN, PH.D. 



(FROM THE ZOOLOGICAL LABORATORY OF THE UNIVERSITY OF PENNSYLVANIA.) 



Plates I-X. 

 [Read April 4, igo2.) 



I. Introduction. 



I. Historical — Although Brachiopoda are chiefly notable be- 

 cause of their great abundance in past geological periods, their 

 relationships to other groups of animals are still so obscure as to 

 make them objects of great interest to the general morphologist. 

 At different times and by different investigators they have been 

 variously regarded as allied to MoUusca, Polyzoa, Annelida, 

 Chaetognatha and Phoronis, while others have regarded them as a 

 distinct phylum of the animal kingdom. Even at the present time 

 there is no uniformly accepted view as to their relationships, all of 

 the above affinities (excepting possibly the first) being maintained 

 by different authors. Of the two morphological methods of inves- 

 tigating relationships, viz., Comparative Anatomy and Comparative 

 Embryology, the former has been applied to this group of animals 

 in a number of noteworthy cases. Not to mention the large num- 

 ber of older and less important works on this group, the comprehen- 

 sive studies of Albany Hancock ('58) and the excellent researches 

 of Lacaze-Duthiers ('61), which are still models of accuracy, the 

 extended labors of Davidson [^Zd-^Z) and most recently the 

 series of splendid contributions by Blochmann ('92 and 1900) have 

 made us as well acquainted with the anatomy of the brachiopods as 

 we are with the anatomy of most other invertebrates. 



The case stands far differently with the embryology of this 

 group. But two writers have as yet attempted to deal with the 

 entire embryology of a brachiopod, and both of these studies were 

 made without the employment of serial sections or modern micro- 

 scopical and micro-technical aids. 



Neglecting the isolated observations of Fritz Miiller ('60 and '61) 

 of a free-swimming larval brachiopod, and the more extended but 

 still very fragmentary observations of Lacaze-Duthiers ('61) on the 

 development of Thecidium, the credit of having made the first 

 study of the entire development of a brachiopod belongs to the 



