62 Proceedings of Eoyal Society of Edinburgh. [sess. 
I fail, however, to understand the latter’s statement that the 
nervous system is “ mainly ventral.” All the main nervous con- 
centrations lie in the atrial floor ; in fact, in or close to the line 
joining mouth and anus. 
Lophophoral Origans . — These appear to he compound organs, con- 
sisting of (1) the ciliated lophophoral nerve ring ; (2) excretory cells 
in a mass at the base of atrium ; (3) ectodermal flap or funnel facing 
inwards, and in contact with the inner row of tentacles. This latter 
consists of ciliated ectoderm inside and atrial epithelium outside ; 
between these is a continuation of the collar body cavity. At the 
base of the funnel is a minute aperture on each side opening into 
the collar cavity. I believe the flap to be the remnant of a dorsal 
lamella, and the pores to be true collar pores. The lophophoral 
organ or flap extends for some way (quite a quarter of a coil in 
P. australis ), and is not short, as has been figured by one observer. 
Body Cavities . — Each of the divisions of the body already referred 
to, namely, epistome, collar, and trunk, has a well-defined coelomic 
cavity, separated from each other by mesenteries. 
Between the coelomic peritoneum and the ectoderm is the 
“ basement tissue,” which is apparently the result of the secretory 
activity of cells which lie imbedded here and there in the matrix, 
and which are probably mesoblastic in origin (Caldwell). This 
skeletal structure does not appear to differ in any essential degree 
from cartilage, and may be directly compared to the “ chondroid 
tissue ” of Balanoglossus. It lines the whole body under the ecto- 
derm and between the coelomic walls. Thus, between the collar 
and trunk cavities, it forms a thick “ septum ” or “ diaphragm,” to 
which are attached the longitudinal muscles. This chondroid tissue 
and the vascular spaces evidently occupy the blastocoelic cavity. 
The tissue is not present to any extent in the very young Phoronis. 
The mesentery between the epistome and the collar supports the 
lophophoral and tentacular veins, but is not thickened by chondroid 
tissue. 
On either side of the branchial fissure each collar cavity has a 
short diverticulum which ends in a minute aperture to the exterior, 
the collar pore. The epistome and its cavity is produced into two 
long processes which run dorsally to the collar cavity in each arm. 
It has a communication by a small pore on each side into the 
