ON THE DIPLOOHORDA. 379 



of posterior coalomic sacs. This cannot possibly have been 

 the case, for Caldwell's embryos were cut in the tentacles of 

 the parent, and in all the species of Phoronis whose de- 

 velopment has been known up till now the larvae leave the 

 parent at the stage with only one pair of tentacles long 

 before the invagination commences. Apart from this, Cald- 

 well's figures, drawn by camera from the sections, scarcely 

 allow of such an explanation. 



Lastly, he refers to my comparison of Phoronis and 

 Balanoglossus, and objects to it on the score of the dif- 

 ferent fate of the blastopore in each case. 



Reference to this will be made later. 



Thus the ontogeny of Phoronis is almost unique in this 

 respect, that with regard to it tliere is universal disagree- 

 ment on such an important point as the development of the 

 mesoderm. 



In a former paper I have investigated the structure of tlie 

 late Actinotrocha larva, but was debarred from following 

 the early stages because neither Phoronis nor the early 

 Actinotrocha occurs in St. Andrews Bay. The later 

 larvfB appear to be brought by the tide currents from else- 

 where, probably from the Frith of Forth, in which Dr. Stret- 

 hill Wright first found the adult. 



Although a renewed investigation of the early stages 

 might fuimish fresh proof of the correctness of the view 

 which was suggested of the natural affinities of Phoronis, 

 and could hardly reveal anything prohibitive of it, more 

 especially as the origin of the mesoblast in Tornaria is still 

 considered as undecided (cf. Spengel and Morgan), it 

 seemed justifiable to publish at once the facts already found, 

 such as the five ccelomic sacs, and draw the legitimate in- 

 ferences from them rather than keep back the whole work 

 for years. Such, however, does not appear to be the opinion 

 of some, to judge by recent criticism, so that I determined 

 to attempt an investigation of the early stages in Phoronis 

 Buskii, although the specimens kindly given me by Pro- 

 fessor Mcintosh were preserved in about 1870, and even 



