1895-96.] Mr Arthur T. Masterman on Phoronis. 
69 
CcELENTERATA. 
Proto-trimetamera -Chsetognatha. 
Annelida. Chordata. 
Arthropoda. 
i 
- Triraetamera. 
1 Polymetamera , a 
diphyletic group 
J (at least). 
I cannot here enter into the ontological resemblances between 
Phoronis and the other Chordata , general resemblances will at once 
suggest themselves. 
We may summarise the steps by which Phoronis has departed 
from the general type of Hemichordata as follows : — - 
1. Assumption of sedentary life, with degeneration of pre-oral 
lobe, carrying with it the atrophy of notochord and pro- 
boscis pore. 
2. A synchronous tentacular hypertrophy of collar region. 
3. A synchronous reduplication of trunk region. 
4. Further reduction of epistome, with growth of tentacles across 
mid-ventral line to form branchial space. 
5. Fusion of base of tentacles, and dorsal coiling of lophophoral 
arms, fusing at their bases with body, and thus forming 
atrial space. 
6. Atrophy of ventral half of collar flap, dorsal half persists as 
dorsal lamella. 
7. Atrophy of gill-slits (if ever present) in correlation to the 
perfection of extra-oral branchial apparatus. 
8. Further hypertrophy in large species of Phoronis , of branchial 
system by further coiling of epistome and collar arms. 
