468 R. M. PACE. 



now formed. The two bauds of tissue apparently budding off 

 from the main mesendodermic mass have already been 

 noticed ; possibly these may represent the true mesoderm, 

 which in that case would appear to develop only late in 

 larval life. Prouho, owing to the fact that he only studied 

 the later stages of larval life in which the stomach had already 

 begun to disappear, was led to regard the whole mesendo- 

 dermic mass as mesoderm, or as mesoderm containing some 

 endodermic cells, but hie himself suggests the necessity of a 

 detailed study of the larva in its earlier stages in order to 

 decide the correctness of his view. 



The Alimentary Canal in Larval Bctoprocta. 



The presence of an alimentary canal in Ectoproct larvEe 

 has been described by Barrels, Vigelius, Repiachoff, Prouho, 

 and Harmer. 



Barrels (2 and 3) at one period regarded larval Ectoprocts 

 as having an alimentary canal, owing to his mistaken supposi- 

 tion that the internal sac represented the stomach. Later on 

 he saw reason to modify his views, but he pointed out and 

 depicted (2, pi. vii, fig. 13) an invagination between the 

 pyriform organ and the internal sac, which he regarded as 

 the rudiment of a pharynx. He therefore believed that many 

 Ectoprocts were originally provided with a digestive tube, 

 and that in cases where no such system is formed the endo- 

 derm arises as in other Bryozoa where the digestive system is 

 better developed, but that later on it degenerates to a mass 

 of yolk spherules filling the interior of the embryo. 



Vigelius (25) stated that in the early larval stages of 

 Bugula, a slight split occurs in the mesendodermic mass 

 filling the interior of the larva ; this split he regards as 

 representing a primitive stomach, which however, plays a 

 purely passive role, and does not open to the exterior. Later 

 on the supposed stomach vanishes, and the eudodcrm forms 

 a simple cell mass. 



Repiachoff (23) describes for Tcndra zoster icola the 



