ON THK DIPLOCHORDA. 313 



in the splanchuopleurc, which will form the blood-sinus and 

 vessels of the adult." I can find no confirmation of this 

 statement ; on the contrary, I find the vascular system a com- 

 pletely closed system of sinuses, bounded in various parts by 

 epiblast, mesoblast, or hypoblast, but in no way connected with 

 the coelomic spaces. 



General Considerations. 



In the foregoing pages has been given a brief account of the 

 structure of the larval Phoronis, the investigation of which 

 was undertaken mainly to assist in deciding the true systematic 

 position of this group. As before stated (24, 25, 26), the facts 

 clearly point to a close genetic relationship with Balano- 

 glossus and Cephalodiscus, especially the latter. 



We therefore have, firstly, to institute a comparison of 

 Actinotrocha and Phoronis with these; and secondly, to 

 see whether, this comparison being held valid, the anatomical 

 facts will throw any further light upon the relationship of 

 these groups collectively to others above and below them in 

 the morphological scale. 



Comparison of Phoronis and Balanoglossus. 



Under this head we may compare (1) Actinotrocha with 

 Balanoglossus, (2) Actinotrocha with Tornaria, and 

 (3) Phoronis with Balanoglossus. 



(1) Actinotrocha and Balanoglossus. 



General Shape of Body. — In each case we have an 

 organism of an elongated shape, and showing clearly external 

 indications of a division into three segments, following one 

 another from before backwards. One of these, the pre-oral 

 lobe, is situated in front of the mouth, and forms a prominent 

 organ overhanging the oral aperture. It is narrowed at the 

 base, presenting a stalk in Balanoglossus, and a contracted 

 portion bounded by the atrial grooves in Actinotrocha (cf. 



