70 Dr. 11, V. Cowlea on the Body-cavliies and 



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

 ination of the internal anatomy of the larvse and adults of this 

 most interesting animal has been made. 



During this time quite a number of papers have been 

 written on the embryology of several species of PTioronis, and 

 these 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 specific difi^erences. 



The above accounts, together with attempts to trace a 

 relationship between Fhoronis and the Chordates, or even the 

 Vertebrates^ tlirough the Acti'notrocha, have stimulated in- 

 vestigation, with the result that within tlie last two years 

 several papers have appeared which practically agree on 

 ceitain important points of special theoretical interest. 



Body-cavities. — Reference to the literature published 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. 



Eoule is the only investigator who considers the ActinO' 

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

 of any mesenteries in the larva of Phoronis Sabatieri, and 

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

 mesenchymatous cells wiiich arise partly from the endoderm 

 and ])artly from the " bandelettes mesoblastiques'''' (nephridial 



1 have been able to examine the Acdnotrochce of P. Sabatieri 

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

 have lound that there is a mesentery present along the line of 

 the tentacles as in other species, 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 

 Kotvalevskii, 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 bilaterally 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 tliat archenteric diverticula exist. He recognizes a 



