650 
BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 
number of the ramified species, the symmetry of the zoaria, and the diminution 
of the adventitious tubes. 
Origin of the zoarium . — The first tube of a zoarium is the ancestrula and its 
lower part is a dilated blisterlike form called the protoecium. It is in this dilation 
that (1) the histolysis of the fixed larva and (2) its replacement by the first normal 
polypide living in the ancestrula occurs. 
Fig. 212. — Origin of zoarium. 
Section of a Fenestella, X 60, cutting 
exactly in the plane of the axis and of 
the zooecial apertures to the right (1-10) . 
The initial zooecium (protoecium) is at 
o; the thickening of the axis (ax) com- 
mences at b' ; the apex of the cone of ex- 
pansion of the colony is at p; the vesi- 
cular tissue (o') above p is of secondary 
origin forming during the mature and 
senile life of the colony. (After Cum- 
ings, 1904.) 
Fig. 213. — Protoecium. 
Zoarium, X 25, of Stomatopora parvi- 
pora new species, showing the orbicular 
protoecium. 
The protoecium is visible on all incrusting species in which the zoarial form 
is that of Proboscina or Berenieea (fig. 213). In the free species it is visible 
only in the section properly made in the base of the zoarium (fig. 212). The 
scarcity of specimens has not allowed us to make a special study x of this feature. 
1 The reader will find excellent models in the studies of Cummings. 
1904. Development of some Paleozoic Bryozoa, American Journal of Science, pp. 49-78, with 83 figures. 
1905. Development of Fenestella, American Journal of Science, pp. 169-178, 3 pis. 
1905. Development and Systematic Position of the Monticuliporoids, Proceedings of the Paleontological 
Society, vol. 23, pp. 357-366, with plates. 
