1895 - 96 .] Mr Masterman on the Structure of Actinotrocha. 135 
Phoronis and of its larval form, Actinotrocha , clearly points to the 
following : — 
Firstly, this genus is closely allied to the members of the Hemi- 
chorclata, and Balanoglossus in particular. 
Secondly, the present structure of Plioronis points to a marked 
degeneration, due to the assumption of a sedentary life, the degree 
of highest organisation being reached in Actinotrocha , just before 
the metamorphosis which results in the transformation to the adult 
condition. 
Thirdly, the structure of Actinotrocha conforms to the hemi- 
chordate type extremely closely, even to minute particulars. 
The evolutionary processes here indicated have an exact parallel 
in the relationship of the Urochorda (or Tunicata) to the Ceplialo- 
chorda. In a precisely similar manner the tailed Urochorda larva 
reaches a certain maximum of structural complexity not far re- 
moved, if perhaps a little above, that of the adult Ampliioxus, and 
then degenerates to the adult Ascidian, with a similar loss of pre- 
oral nervous system, sense organs, and notochord. 
Perhaps the main difference in the two processes is this, that the 
larval Ascidian becomes fixed by the pre-oral lobe, as also in, for 
example, the Echinodermata ; whereas the Actinotrocha becomes 
fixed by that organ which is probably homologous with the bud- 
ding stolon of Cephalocliscus, and the permanently attaching cord 
of Rliabdopleura. This ontogenetic difference is also probably of 
phylogenetic value, and would largely explain the differing types 
of metamorphosis. 
In conclusion, I wmulcl suggest that these relationships justify 
the classification of Phoronis as a separate division of the Chordata , 
which may be termed the Diplocliorda , so that the Chordata , as 
thus constituted, would be as follows : — 
Chordata — A. Trimetamera. 
1 . Diplocliorda — Phoronis. 
2. Hemichorcla — Balanoglossus, Cephcdodiscus, 
Rliabdopleura. 
B. Polymetamera. 
3. Urochorda — Ascidici, &c. 
4. Cephalochorda— Ampliioxus. 
5. Holochorda — Pisces , Amphibia, &c. 
