STUDIES ON THE PEOTOCHOBDATA. 339 



are derived essentially from ectodermic invagina- 

 tions; and the idea that the visceral wall of the atrium is 

 derived from the hypoblast, as put forward by van Beneden 

 and Juliu, is entirely without foundation. It may be added, 

 further, that the expression "primary branchial canal," which 

 van Beneden and Julin employ for the canal formed by the 

 fusion of the floor of the atrial invagination on each side with 

 the wall of the branchial sac to form the first pair of stigmata 

 in Clavelina, has no morphological meaning, 



On account of the crucial importance of a correct apprecia- 

 tion of the atrial cavities from the point of view of the relations 

 between the Urochorda and Cephalochorda it is necessary to 

 make a somewhat lengthy quotation from the work of van 

 Beneden and Julin (7). They say (p. 402, et seq.), with refer- 

 ence to the formation of the atrial cavities in Clavelina, " In- 



dependarament des invaginations epiblastiques il se 



forme, chez la Claveline, des culs-de-sac hypoblastiques qui 

 procedent de la voute du sac branchial, se soudent aux invagi- 

 nations epiblastiques et se raettent en communication avec 



elles Les culs-de-sac epiblastiques et les diverticules 



hypoblastiques interviennent concurremment dans la formation 

 des canaux branchiaux de la larve urodele. Ces canaux sont 

 evidemment homologues a ceux des Appendiculaires. . . , . 

 Les deux ebauches qui cooperent a la formation d^un canal 

 primitif, interviennent I'une et Fautre dans la formation des 

 cavites peribranchiales ; il est difficile de 'dire dans quelles 



limites Nous pensons que le feuillet visceral de la 



membrane peribranchiale .... est en grande partie d'origine 

 hypoblastique." 



Further on they go on to say that it is doubtful whether 

 these "primary branchial canals" persist and function with 

 the rest of the stigmata, or whether they close up as they do 

 in the buds, where the peribranchial cavities arise by constric- 

 tion from the inner vesicle of the bud. They incline to the 

 latter view. 



In the first place, it follows clearly, from the description I 

 have given above of the origin of the gill-slits in Ciona, that 



