THELACEROS RHIZOPHORA. 553 
position, and had so entirely pushed upwards the stomodeum 
that its inner or lower rim protruded at the oral aperture, 
tightly pressed against the oral rim. 
The oral disc and the tentacles and the column were covered 
by a thin but tough layer, which had peeled off in some places, 
and which consisted of thread-cells, mucus, and adventitious 
particles. Beneath this layer the mesenteries appeared very 
beautifully through the oral disc as strong radial lines. The 
attachment of the mesenteries to the column and the pedal disc 
was visible in places, but was not an obvious feature of the 
external anatomy. The column was quite smooth; there were 
no warts, knobs, or ridges, and there were no cinclides. There 
were no marginal spherules. The colour of the spirit specimen 
was a light fawn on the oral disc and tentacles, ana a dark 
colour on the column and base. 
Oral Dise and Tentacles.—Thelaceros rhizophore 
possesses ninety-six well-marked tentacles, forming a compact 
wreath round the margin of the oral disc. These tentacles are 
arranged in three circles, and they are all approximately of 
the same length. There is, however, evidence of a process of 
centripetal change in position, for the inner and oldest circle 
is distinctly separated from the next circle. This inner circle 
consists of twelve tentacles, equal in size and arranged at 
regular intervals. Six of these communicate with the endo- 
celes of the primary pairs of mesenteries, and six with the 
endoceeles of the six pairs of secondary mesenteries. 
Next comes a circle of thirty-six tentacles arranged in well- 
marked groups of three, between the tentacles of the inner 
row and a little exterior to them. Of these threes the central 
tentacle is placed slightly internal to the other two, and the 
twelve centrals belong to the endoceles of the mesenteries 
of the third order. The twenty-four tentacles which complete 
.the twelve groups of three communicate with the endocceles of 
the fourth order. 
There is an outer circle of forty-eight tentacles placed at the 
extreme edge of the disc, and this outer row corresponds to the 
forty-eight exoceeles. If these exoccelic tentacles be called 0 
