ON THE DTPLOCHORDA. 285 



notrocha with the Heraichorda (24 and 25). In this 

 comparison it was assumed that the struoture of Balano- 

 glossus had been modified by its peculiar habits in such a 

 way that the notochord and other organs had become secon- 

 darily shifted into the pre-oral lobe. This led to the difficulty 

 that the "notochord" of Cephalodiscus (Harmer^ 10) was 

 in the same position, and yet the burrowing habit is not in this 

 case indulged in. 



The discovery of the " subneural gland " (see below) in 

 Actinotrocha, however, seemed to complicate the difficulty; 

 for this organ, although epiblastic, occupies the same position 

 and has the same relationships to other organs as the " noto- 

 chord " of Cephalodiscus. The natural conclusion was that 

 the "notochord " of Cephalodiscus was a subneural gland, 

 and the notochord itself was yet to be found. With such an end 

 in view I examined Cephalodiscus by sections, and there can 

 be no doubt that this animal possesses a paired notochord on 

 the dorso-lateral part of the pharynx. In the light of this 

 and other facts given below, granting that the structural and 

 phyletic relationships of animals are to be expressed in our 

 classification, it seems reasonable, and indeed neces- 

 sary, that Phoronis and Cephalodiscus be included 

 in one group, Diplochorda; whilst their relation- 

 ship to Balanoglossus (or Hemichorda) may be ex- 

 pressed by including the two groups under the name 

 Archichorda. 



The present paper will therefore be confined to an account 

 of the structure of Actinotrocha_, and a note upon the 

 structure of Cephalodiscus. Part I. — The Structure of 

 Actinotrocha. Part II. — The Anatomy of Cephalodiscus. 



