ISO 
Proceedings of Royal Society of Edinburgh . 
SESS. 
the remark — “ I cannot help suspecting that subsequent investiga- 
tions will reveal some trace of an organ homologous to the noto- 
chord in the Actinotroclia stage.” 
A preliminary examination of spirit specimens of Actinotroclia 
has justified these suspicions in such a remarkable degree that I 
here purpose giving a short account of the results. 
Not only does Actinotroclia possess structures which are in- 
dubitably of the morphological value of a notochord, but it also 
presents other features which, I think, must settle the affinity of 
Phoronis to be with the Hemichorda. As is well known, Actino- 
troclia has a large overhanging pre-oral lobe and a somewhat 
elongated body. Behind the mouth is a double row of tentacles, 
which form an almost complete post-oral ring. 
A partial ring of ciliated cells follows the edge of the pre-oral 
lobe, a post-oral ciliated band takes a sinuous course up and down 
the tentacles, and a third ciliated ring surrounds the anus at the 
posterior end of the body, usually called the peri-anal band. 
Actinotroclia is thus very little modified from a type of larva with 
three ciliated rings, the pre-oral, the post-oral, and the peri-anal, 
which correspond, as will be later shown, to the three segments of 
the mesoderm. 
At the front end of the body is a thickened plate of ectodermal 
nervous tissue, usually termed the apical plate. The coelome is 
divided into five pouches or cavities, one unpaired, filling up the 
cavity of the pre-oral lobe, two paired cavities lying immediately 
behind the mouth and produced into the tentacles, and two posterior 
cavities filling up the trunk. 
At about the level of the mouth ventrally, and stretching 
backwards to meet the dorsal ectoderm just in front of the apical 
plate, is a mesentery limiting the pre-oral cavity posteriorly. 
In the immediate neighbourhood of the oesophagus this mesen- 
tery envelopes the blind end of the dorsal blood-vessel, and its 
walls are glandular, thus forming an organ similar to that 
formed in Balanoglossus. I cannot, however, speak with absolute 
certainty with regard to this point, for there are other structures 
connected with the pre-oral lobe which require elucidation by 
examination of fresh specimens. The paired post-oral or collar 
pouches are separated from the trunk cavities by paired me sen- 
