60 Proceedings of Royal Society of Edinburgh. [sess. 
in two parallel rows, an outer and an inner, the inner being on the 
concave side of the coiled lophophore and the outer on the convex. 
Between these two rows of tentacles is contained an imperfectly 
shut-off cavity, which we may term the “ branchial space.” Its roof 
is open at the apex of the tentacles, its lateral walls are formed by 
the tentacles, and its floor is formed partly by the epistome, and, 
partly on the outer side of it, the mouth. The inner row of 
tentacles is in close contact with the epistome throughout this 
cavity, except in the middle line. Here the tentacles are dis- 
continuous for a small space, those of each side, however, overlap- 
ping so that a “ branchial fissure ” is produced which leads from 
the branchial space into the lophophoral concavity. 
The lophophoral arms are coiled round so that the inner row of 
tentacles also tend to inclose a space which we may term the 
“ atrial space.” This opens freely above, has the outer wall of the 
collar below, and is closed in by the inner row of tentacles, except 
in the median line. Here it opens orally to the branchial fissure 
and aborally to the exterior. 
Into the atrial space are discharged the waste products from the 
anus, nephridia (including sexual products), and lophophoral organs. 
Both branchial and atrial spaces in the large species of Phoronis 
are continued up the three coils of the arms. 
Ectoderm . — The ectoderm consists, for the most part, of three 
types of one-layered epithelium. 
The trunk has numerous circular transverse furrows which are 
found to be due to a plication of the epithelium. This is a per- 
manent and normal folding of the glandular cuticle-secreting cells, 
and is not, as formerly asserted, due to contraction during the pro- 
cesses of killing and preserving. 
In the collar and epistomial regions there occur the two other 
types of epithelium. The first or branchial has very much elongated 
glandular ciliated cells, with small nuclei ; they are found covering 
the whole surface of the branchial space, as defined above, and are 
continuous with others of a similar type lining the oesophagus. These 
cells secrete copious mucus, strings of which are noticed in section 
to he passing down the tentacles and into the mouth, carrying 
numerous food particles adhering to them. This method of food 
ingestion forcibly recalls that of the Tunicata. The epistome pro- 
