ON THK DlFLOOHOliDA. 319 



showu iu coronal sections of the two types (ti^'s. 50 and 51). 

 In each case the organ has precisely similar relationships to the 

 pre-oral coelonij collar ccelom, and subneural sinus (or heart), 

 though there is a striking difference in the fact that the sub- 

 neural gland has a duct to the exterior, whilst the proboscis 

 vesicle has none in the adult, tliough it arises in close contact 

 with the epiblast, even if not actually from this layer. We 

 have assumed a forward movementof the notochord in Balano- 

 glossus, and this movement must have considerably altered 

 the environment of the proboscis vesicle. It is not incon- 

 ceivable that this organ might be carried forwards and lose its 

 connection with the exterior, as is shown in fig. 48. 



The subneural gland is undoubtedly epiblastic in origin, 

 and arises in the mid-ventral line, whereas there is considerable 

 diflFerence of opinion regarding the origin of the proboscis 

 vesicle. Spengel (19a) maintains that it is epiblastic in origin, 

 and Morgan (17) is inclined to believe that it is mesoblastic. 

 Morgan's figures show the early rudiment of the vesicle in 

 close contact with the epiblast ; and I think that although 

 Morgan (17) interprets the organ as formed from mesenchyma, 

 there is as much testimony in Jiis figures to its epiblastic 

 origin as is found in the case of the segmental duct of Verte- 

 brata and the nerve-cord ofTeleostei. Although the organ 

 in Actinotrocha arises ventrally, it comes into connection 

 with the dorsal sinus and other parts iu such a way that it 

 would not be surprising to find the mid-ventral invagination 

 method iu this type altered to the modified process of develop- 

 ment in Balanoglossus. With the forward movementof the 

 notochord a mid-ventral development of the proboscis vesicle 

 would be an ontogenetic process of great difficulty. (We might 

 here compare Willey's [23] suggestion of the alteration in posi- 

 tion of the mouth of Amphioxus, correlated with a forward 

 growth of the notochord.) The structure of Cephalodiscus 

 is especially instructive with regard to the subneural gland. 



Gill-slits. — I have not as yet been able to find any trace 

 of gilUslits or their homologues in Phoronis or Actino- 

 trocha, though their analogues are present in each case, 



VOL. 40, PART 2. — NEW SER. Z 



