50 CONKLIN — EMBRYOLOGY OF A BRACHIOPOD. [April 4, 



enteron and two lateral ones which form the coelom. It is diffi- 

 cult to determine from Kowalevsky's figures from which portion 

 of the archenteron these lateral partitions arise, but there can be 

 no doubt that they are regarded as folds of the archenteric wall 

 (see his Fig. 6), nor that they divide the archenteron into three 

 cavities, the enteron and two coelom sacs. In both of these 

 respects Cistella is very unlike Terebratulina ; in the latter the 

 archenteron is first divided into two cavities and not three (the 

 enterocoel is for a long time unpaired), and the single partition wall 

 by which this division is brought about consists of a single layer of 

 cells and not a plication of the archenteric wall (though in its 

 earliest stages this partition wall probably represents such a plica- 

 tion. Fig. 42«). The former of these differences is perhaps not 

 so great as would at first thought appear, being principally due 

 to the fact that in Terebratulina the -enteron occupies but a small 

 part of the archenteron, and hence the partition wall which 

 separates it from the enterocoel leaves the latter a large unpaired 

 cavity, whereas in Cistella the division of the archenteron is 

 more nearly equal and when completed separates two lateral 

 enterocoel pouches from the median enteron. But one cannot 

 overlook the fact that according to Kowalevsky two partition 

 walls are formed in Cistella, whereas but a single one is found in 

 Terebratulina. Moreover these partitions are lateral in position 

 according to Kowalevsky, whereas in Terebratulina the single par- 

 tition grows out from the anterior wall and merely curves around on 

 to the lateral walls of the archenteron (Fig. i6). The difference 

 in the structure of this partition in Cistella and Terebratulina is 

 also important \ in the former it consists of a double layer of cells, 

 in the latter of a single layer ; in Cistella the wall of the enterocoel 

 sacs lying next to the enteron becomes the splanchnic layer of the 

 mesoblast, in Terebratulina the splanchnic mesoblast is derived 

 from mesenchyme cells. Before attempting to explain these im- 

 portant differences between Cistella and Terebratulina in the 

 formation of the coelom it would be well to know that they actually 

 exist, and it seems highly desirable that the embryology of Cistella 

 should be reinvestigated with the aid of modern histological 

 methods. 



The differences between Terebratulina and Sagitta in the mode 

 of forming the coelom are of interest since they remove one im- 

 portant argument for the supposed relationship between Brachio- 

 poda and Chaetognatha. 



