i»5 
inclination of the oral cone and through the fusion of mesenteries with actinopharyngeal folds. 
Each pair of primary sagittal mesenteries is attached to such folds in two places (PI. V, fig. n). 
At a lower level in the oral cone the mesentery is loosened from one of these folds but fixed 
on a second place to the bodywall at the base of the sagittal tentacle (PI. V, fig. 13). In this 
same fig. 13 the upper part of a blind interseptal room is figured (/. r 1 ), the wall of which is 
formed by the upper part of a secondary mesentery towards the side of the gastral cavity, 
this secondary mesentery in its upper part is not extended between bodywall and actinopharynx, 
but between two folds of the actinopharynx. At a still lower level (PI. V, fig. 14) this secondary 
septum is fixed to the bodywall also. The secondary septum does not extend far enough upwards 
to reach the top of the oral cone. — The mesenteries consist of a thin mesogloeal lamella and 
entoderm of varying thickness, and of the same structure as the actinopharyngeal entoderm. 
The primary transversal mesenteries and the secondary mesenteries have unilateral mesogloeal 
lamellae (PI. V, fig. 10). One of these mesenteries is enlarged in PI. V, fig. 23, and one of 
the secondary ones in PI. I, fig. 8. These mesogloeal lamellae are continued by the bases of 
the entodermal cells; dark polished cross-sections of longitudinal musclefibres are arranged in 
a regular manner against these lamellae (PI. I, fig. 8). These musclefibres are entirely absent 
at the other side of the mesenteries. — None of the many series of sections gave a very clear 
insight in this musclesystem; the longitudinal muscles are found on those sides of the secondary 
mesenteries, which are averted from the primary transversal ones, but I am not very sure of 
the fact that these fibres also occur on the primary sagittal mesenteries, on the side averted 
from the transversal ones. 
Reproductive organs. All the polyps contain ovaria, but no testes; the colony is probably 
not hermaphrodite. The ova are found in the primary transversal mesenteries only in such large 
numbers that they fill almost the entire gastral cavity. The mesenterial mesogloea, which is 
easily to be distinguished in that part of the mesentery which is nearest the bodywall, is 
dissolved in the fertile part into a quantity of undefinable fibres, so that the ovaria have no 
clearly defined mesogloeal capsule. In some places parts of the mesogloea are visible (PI. I, 
fig. 7). The mesentery is enormously swollen by the ova. The ova (PI. I, fig. 6) are heavily 
laden with yolk. The protoplasma is gathered round the large nucleus, which contains a large 
nucleolus, often with some smaller ones. Although the shape of the ova may be so much 
influenced by plasmolyse that there is a semblance of an egg-cone, as is described by Hertwig 
for the Actinia, such an egg-structure was not observed with certainty. The swift resolving of 
the mesogloea renders it impossible to make out in what manner the primordial germ-cells are 
encased in the mesoodoea. 
o 
The mesenterial filaments (PI. I, fig. 11) are found along the primary transversal 
mesenteries only. They are repeatedly branched (PI. V, fig. 16) so as to fill a large part of 
the lower half of the gastral cavity. They are immediately to be recognised by the typical, 
brown pigmentation, which is very dense here and which was also present in the actinopharynx- 
ectoderm. The structure of the ectoderm is the same as in the actinopharynx. I he mesogloea of 
the mesentery is broadened at the base of the filament and contains oval connective tissue cells. 
This broadened part is forked in two branches, which are at a very wide angle one with the 
SIBOGA-EXPEDITIE XVII. 
24 
