130 WILLIAM BLAXLAND BENHAM. 
membrane,” the skeleton is incomplete. The basement tissue 
of the outer and inner surfaces is connected by vertical 
trabecule of the same tissue, placed at regular intervals, thus 
dividing this part of the colom into a number of separate 
cavities—the tentacular cavities. A little farther from the 
lophophore, an additional layer is seen on each side of each 
trabecula: still further upwards, this skeletal layer on each 
side of a tentacular cavity becomes thicker and is connected 
by a similar layer round its outer surface, so that now there is 
a semicircular skeleton resting on the basement tissue (fig. 14). 
When the “tentacular membrane” is passed the basement 
tissue forming the trabeculz has disappeared, and the skeletal 
layer has extended all round the cavity, as seen in fig. 13. 
There is not, therefore, a true “membrane” uniting the 
tentacles, asin Plumatella, for instance; but the tentacles, 
which are apposed to one another, are merely connected 
together by the trabecule of basement tissue. t 
In connection with the tentacular crown two structures 
must here be mentioned which have received various interpre- 
tations. 
At the base of the inner series of tentacles there is a pit on 
the dorsal surface on each side (fig. 7, gl.’). It was first 
observed by Dyster, and has been described by Caldwell (4) 
and McIntosh (19) as a “ ciliated pit ” having a sensory func- 
tion. I find no reason for regarding it as sensory ; it has not 
the structure which we should expect to see in a sensory 
organ, and I can detect no nerve going to it. 
Its structure is rather that of a gland, and McIntosh mentions 
that he observed mucus in the pit, and suggests that it may 
combine a glandular with a sensory character. Its outer wall 
consists of several layers of cells, the nuclei of which stain 
deeply (fig. 8, f.). The outermost layer consists of short 
columnar cells, which are continuous with the somewhat 
cubical cells of the epidermis of this region. The cells of the 
innermost layer, facing the pit, carry short cilia, and these are 
also found at the bottom of the pit, and for a short distance 
along its inner wall. But the greater part of the inner wall of 
