ON THE DIPLOCHORDA. 315 



ring are given off fibres forming a ventral nervous tract in the 

 trunk. In addition there is a mid dorsal nervous tract along 

 the trunk, concentrated into a cord in Balanoglossus, in a 

 more diffused condition in Actinotrocha. Lastly, there is 

 the perianal nervous ring in the latter. 



It is difficult to follow this comparison with the semi-dia- 

 grammatic figures (figs. 46, 47), and to account for the resem- 

 blances as accidental or homoplastic. 



Alimentary System. — We have seen that there is some 

 reason for suspecting the presence of a stomodaeum in Acti- 

 notrocha, whereas, so far as evidence goes, such is scarcely 

 present in Balanoglossus. Again, the hypoi)lastic system 

 is complicated in the latter, by a forward movement of certain 

 of the organs, into the proboscis, giving them a secondary 

 pre-oral position. Thus it cannot be denied that the noto- 

 chord is secondarily displaced into the proboscis, probably in 

 connection with a burrowing habit. The proboscis is pri- 

 marily pre-oral, and therefore de facto has no hypoblast in its 

 cavity ; any hypoblastic organ must, therefore, invade it second- 

 arily. Thus Bateson states that the notochord arises in the 

 collar region and moves forward later in development. 



This moving forward phyletically of the notochord must 

 necessarily involve the displacement of other structures, and 

 together with the absence of a stomodseum must be allowed 

 for in our comparison. 



At the front end of the pharynx in Actinotrocha are a 

 pair of organs which have been described and referred to above 

 as '^notochords.'^ 



The presence of a "notochord" has always been so insisted 

 upon as an essential character of the Chord ata that at first 

 sight one might be led to suppose that the proving of the 

 identity of these organs of Actinotrocha with the notochord 

 of Balanoglossus is a sine qua n on for the assumption of 

 their close genetic connection. This does not, however, 

 appear to be the case, for the anatomical likeness is so marked, 

 quite apart from the " notochord " question, that it merely 

 serves to form an additional reason for regarding these latter as 



