70 Dr. R. r. Cowlos on the Boilij-cavit'ws and 



the last twenty-five years that a more or less careful exani- 

 ii)ation of the interntil anatomy of the larvae and adults of this 

 most interesting animal has becMi made. 



During this time quite a number of papers have been 

 •written on the embryology of several species of Phoronis, and 

 tiiese are remarkable for the great disagreements concerning 

 certain fundamental facts in the development. However, the 

 conflicting nature of these descriptions, it seems to me, is 

 probably very largely due to sj)ceitic differences. 



The above accounts, together with attempts to trace a 

 relationshij) between Phoronis and the Chordates, or even the 

 Vertebrates, through the Actinotrucha, have stimulated in- 

 vestigation, with the result that within the last two years 

 several papers have appeared which practically agree on 

 ceitain imj)ortant points of special theoretical interest. 



Body-covities, — lleference to the literature publislied on 

 the larval body-cavities of the Actinotrocha shows that our 

 knowledge is in a very perplexing state. One investigator 

 finds a single body-cavity, others tell us there are two, another 

 describes three body-cavities, and one worker claims that 

 there are five. 



Roule is the only investigator who considers the Aciino- 

 trccha to have but oiie body-cavity. He denies the presence 

 of any mesenteries in the larva of Phoronis Sahatieri, and 

 says that the lining of the body- cavity has its origin from 

 mesenchymatous cells which arise partly from the endoderni 

 and partly from the " bandelettes mesoblastiques^'' (nephridial 



1 have been able to examine the Actinotrochce of P. Sahatieri 

 through the kindness of M. Marc de S^lys-Longchamps, and 

 have louiid that there is a mesentery ])resent along the line of 

 the tentacles as in otherspecies, but that it is not very highly 

 developed. With the material at hand I am unable to give 

 any opinion as to the presence of a mesentery anterior to 

 this one. 



Caldwell, from a study of the development of Phoronis 

 Kowalevskii, finds that the Actinotrocha has but two body- 

 cavities, separated from one another by a mesentery which is 

 inserted along the line of the bases of the tentacles. This 

 investigator claims that the mesoderm arises partly from 

 archenteric diverticula bihitcrally placed immediately back of 

 the blastopore, partly from the walls of the archenteron 

 posterior to this, and partly from the posterior pits (nephridial 

 pit), which he considers to be of endodermal origin. 



Longchamps, who has recently studied the same form, 

 denies that archenteric diverticula exist. He recojniizes ii 



