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



teron from its apex, thus dividing the cavity into a median one, the 

 enteron and two lateral ones, the enterocoels. The stomodseum 

 is said to open into the apical end of the enteron, directly opposite 

 the blastopore, which has however closed at an earlier period. In 

 Terebratulina, on the other hand, the enteron is separated from the 

 single enteroccel by one partition which grows out from the anterior 

 wall of the archenteron and divides the latter into a dorsal cavity, 

 the enteron, and a ventral one, the enteroccel ; only later, by the 

 closure of the blastopore and the flattening of the embryo, does the 

 enteroccel become divided through its middle region into right and 

 left cavities, which, however, continue for some time to communi- 

 cate with each other both anteriorly and posteriorly. Moreover the 

 stomodaeum in Terebratulina is formed in the position of the for- 

 mer blastopore and not on the opposite side of the embryo. There- 

 fore, although there are certain general resemblances between the 

 two, I cannot regard the coelom formation in chaetognaths and 

 brachiopods as being more than analogous processes, and as such 

 devoid of phylogenetic significance. 



In Phoroms, according to Caldwell ('85), two pairs of coelomic 

 cavities are formed by a modified type of enteroccel formation, 

 which however bears no resemblance to that in Terebratulina. 

 The anterior one of these cavities gives rise to the cavities of 

 the epistome and lophophore, the posterior one to the chief body 

 cavity. 



Masterman's (1900) work on the development of Phoronis fur- 

 nishes the most complete account of the early development of this 

 interesting form which has yet been given. ^ In an earlier work 

 ('97) he found that there were three separate and distinct coelomic 

 cavities in the larva, a preoral or epistomal cavity, a collar or lopho- 

 phoral cavity and a trunk cavity. In his later paper he describes 

 the origin of these cavities ; the first of these arises as a median 

 outgrowth from the anterior side of the archenteron, the other two 

 arise as paired masses of solid mesoderm cells in which cavities 

 appear later. In the matter of the formation of the anterior or 

 procoelomic cavity there is much resemblance between Phoronis and 

 Terebratulina^ but in the latter animal the mesocoelomic and meta- 

 coelomic cavities are entirely absent. 



1 Since the appearance of Ideka's (1901) work this statement is no longer 

 true. Ideka has given by all odds the most complete account of the embryology 

 of Phoronis yet published. (See postscript, p. 70.) 



