442 MORGAN. [Vol. V. 



enteroccel opens in both forms by a water pore on the dorsal sur- 

 face a little to the left of the middle line. We have good reason 

 to believe that the ancestor of Tornaria had two of these dorsal 

 openings — right and left. This is indicated in the adult of B. 

 Kupfferiy and I have found it in a single specimen of B. Kow- 

 alevski when its presence was presumably atavistic. Moreover, 

 Cephalodiscus, which is undoubtedly related to Balanoglossus, has 

 a pair of these dorsal water pores. It is also of the greatest 

 importance that when these two openings are present they both 

 communicate with the single unpaired enterocoel. In the Auric- 

 ularia there has been discovered in some cases at least a pair 

 of these dorsal pores, opening on the dorsal surface right and 

 left and communicating with the single anterior enteroccel, as 

 pointed out by Ludwig and more recently by Brooks and Field. 

 Subsequently the right pore disappears, and the left remains, 

 resulting in the condition found in Tornaria. The formation of 

 muscles from the cells of the inner wall of this enterocoel is 

 common to the two groups. Finally, an intimate connection 

 between a part of this enteroccel and the so-called heart or 

 anterior blood-vessel is found in both Balanoglossus and Echino- 

 derm. The presence in both Tornaria and Auricularia (as 

 shown by Field) of a mesenchymatous vesicle closely connected 

 with the anterior enteroccel, and in the one case certainly, and 

 in the other presumably, also connected with the formation of 

 the heart, is most significant. The presence of wandering 

 mesenchyme in the blastoccel is common to the two ; but more 

 important is the accumulation of many of these cells around the 

 endodermal oesophagus. This can hardly be explained by mere 

 chance. The oesophagus itself is almost entirely endodermal 

 in Tornaria, as is indicated by the shortness of the stomodaeal 

 invagination in the larva studied by Bateson, and in Tornaria 

 by the formation of gill pouches and " notochord " from its walls. 

 The oesophagus of Auricularia is also almost entirely endoder- 

 mal in its origin. The division of the digestive tract into three 

 compartments is the same in each, although this is hardly of 

 great importance. The course of the longitudinal ciliated 

 band is very similar in the two ; indeed, the one might almost 

 be substituted for the other ; and if I have been fortunate in my 

 comparison of a breaking at the apical plate and of a secondary 

 tendency of a new fusion from right to left, we see an exactly 



