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 aoarium. 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. FIG. 213. Protoecium. 



Section of a Fenestella, X 60, cutting Zoarium, X 25, of Stonwtopora parvi- 



exactly in the plane of the axis and of pora new species, showing the orbicular 

 the zooecial apertures to the right (1-10) . protoecium. 



The initial zooecium (protoecium) is at 

 o; the thickening of the axis (ax) com- 

 mences at &'; the apex of the cone of ex- 

 pansion of the colony is at p; the vesi- 

 cular tissue (c') above p is of secondary 

 origin forming during the mature and 

 senile life of the colony. (After Gum- 

 ings, 1904.) 



The protoecium is visible on all incrusting species in which the zoarial form 

 is that of Probosdna or Berenicea (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 t of this feature. 



'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. 169178, 3 pis. 



1905. Development and Systematic Position of the Monticuliporoids, Proceedings of the Paleontological 

 Society, vol. 23, pp. 357-366, with plates. 



