ON THE DIPLOCHORDA. 405 



that the wall separating the trunk somites from the archen- 

 teron is last closed on the dorsal side, and in Ph or on is the 

 trunk somites are dorso-lateral in origin, whilst his section 

 (fig. 83) shows the collar somites to be ventro-lateral, in this 

 respect also resembling the homologous structures in Pho- 

 ronis. 



It will thus be seen that the formation and relationships of 

 the several parts of the mesoblastin Phoronis resemble even 

 in minute particulars that of Balanoglossus, the resem- 

 blance in mode of origin being rather closer to Torn aria 

 than to the demersal type, though in many respects the latter 

 differs more from Tornaria than from Phoronis. 



My comparison of Actinotrocha with Balanoglossus 

 and Cep ha discus receives very little modification with 

 further work upon the subject. Professor L. Roule (15a) 

 has published one or more papers in which he states that he 

 is unable to find in the Actinotrocha of Phoronis 

 sabatieri certain structures which I have described in the 

 Actinotrocha at St. Andrews; perhaps further research 

 may clear up this difficulty. He corroborates the descrip- 

 tion given of the chordoid structures in Actinotrocha, but 

 is inclined to regard this larva as closely allied to the tro- 

 chophore of the Annelida. My views of the relationship 

 of the trochophore to Actinotrocha are sufficiently clearly 

 indicated in a recent paper (10a), but it may be noted here 

 that Roule makes no reference to the live coelomic cavities 

 which I have figured and described in Actinotrocha, 

 though the mesenteries separating them can be seen with a 

 hand lens in the living or mounted larva ! ^ His paper has no 

 figures, and his methods are not described. Roule's account 

 of the early development of the mesoblast in Phoronis so 

 differs from mine and from that of others that it is not sur- 

 prising that there should be a like discrepancy in description 

 of the later stages. 



' As already noted (' Quart. Journ. Mier. Sol.,' August, 1897), they were 

 correctly figured by R. Wagener as early as 1847, but were supposed by liim 

 to be nerves. — A. T. M. 



