554 OIJESSWKLL SIIKARKJ;. 



Kofsclielt (22) lias called attention to the relation of the 

 ecto- and coelomesoblast in Pliysa. If we take a section of 

 such a stage of Physa as is shown in Text-fig. 11, he points 

 out that the ecto- and coelomesoblast between them form a 

 complete ring round the blastopore. He thinks this condition 

 points to the conclusion that in Annelids and Molluscs ecto- 

 and coelomesoblast were oi'iginally one structure, which has 

 been divided and specialised as the result of larval develop- 

 ment. In Phoronis and the other great group of animals 

 of the Deutei'ostomia type this has not taken place. 

 Phoronis is undoubtedly closely related to the Annelids in the 

 Actinotrocha stage, with its solenocyte-bearing uephridia and 

 ciliated rings, but shows no segregation of the coelomesoblast 

 into pole-cells. 



From the work of De 8elys Longchamps (34) we know that 

 the mesoderm consists of a large number of irregular cells 

 scattered throughout the blastocoel. I have shown (36), and 

 it has also been clearly demonstrated by the work of other 

 investigators, that these cells arise in the region of the 

 blastopore, or from the line along which the blastopore has 

 ali-eady closed. The cells resemble the larval mesenchyme 

 of Annelids more than the cells of the Annelid ccelomeso- 

 blast. 



In Brachiopods the mesoderm is also of the irregular 

 variety, and arises from the ccelom, which is here a direct 

 outgrowth from the anterior end of the primitive archenteron, 

 as Conklin (7) has recenly desci-ibed in Terebratulina. 

 No division into ecto- and coelomesoblast can be distin- 

 guished, and it is purely coelomesoblastic. 



There would thus seem to be a sharp division between 

 Phoronis and Brachiopods on the one hand, and Annelids 

 and Molluscs on the other. In one we get a sharp division 

 of the mesoderm into two portions, while iu the other there is 

 no such division. Korschelt (22) thinks that without a more 

 definite knowledo-e of how the coelomesoblast arose iu the 

 hypothetical Annelids, we cannot reconcile these two types 

 of mesoderm formation. 



