ON THE DIPLOCHORDA. 283 



The conteuts of the following pages are the result of more 

 or less intermittent work since 1894, which has led me to hold 

 that, although Phoronis has some features in common with 

 both Gephyrea and Polyzoa, its more immediate genetic 

 allies are not usually found in either of these groups, but are 

 commonly placed together under the title Hemichorda. 



Thus whilst the more pronounced community of structure 

 holds between Phoronis and Cephalodiscus, yet the like- 

 ness of the former to Balanoglossus in certain other cha- 

 racters is so marked that it rather tends to strengthen the 

 alliance between the two latter. 



No one can doubt that Phoronis is, in the adult stage, 

 greatly specialised for a sedentary life, and that this specialisa- 

 tion is, as in the case of the Tunicata, a degeneration involving 

 a loss of certain organs, degradation of others to a lower 

 structure, and yet again hypertrophy of others which are not 

 so prominent a feature in the archaic type. Thus a study of 

 the adult alone, with a view to phyletic conclusions, would be 

 embarrassed by these attendant complications ; and were 

 Phoronis to reproduce itself by a fcetal method, these could 

 not be surmounted. Fortunately, in Actinotrocha we have 

 a free larval form, which not only must be construed, with 

 certain reservations, as exemplifying the structure of the 

 undegenerate free-swimming ancestor of Phoronis, but which 

 has a very well-marked culminating point in differentiation, 

 with a sudden decline in morphological status to the adult 

 condition. This being so, an investigation of the fully 

 developed Actinotrocha should offer the readiest solution of 

 the problem before us. 



In Part I, I have gone over the whole structure of the 

 Actinotrocha larva, as can be discerned by an examination 

 of preserved specimens, both whole and in section. The 

 method of examination by serial sections has enabled me to 

 correct some misconceptions with regard to the structure of 

 this unique larval form. It is purposed to show that Actino- 

 trocha has so close a similarity in structure to the three 

 members of the Hemichorda that the assumption of a 



