NORTH AMERICAN EARLY TERTIARY BRYOZOA. 
45 
of the ectoderm results in the ectodermal groove. Near by this groove a mass of 
cells, the 'pyriform organ ( p .), forms. Another larval structure, the calotte or 
retractile disk (disk) provided with motionless sensory cilia, develops at the 
opposite or aboral pole. 
The larva now escapes from the ovieell and commences a short period of free 
life. Up to this time an alimentary canal is absent, but now changes occur which 
result in a complete metamorphosis of the larva. By a contraction of the body 
the sucker is turned inside out and affixes the larva to some foreign object. The 
aboral side, containing the retractile disk and pallial groove, becomes expanded 
and is the source of the entire outer covering of the primary zooicl. This expan- 
sion obliterates the pallial groove and at the same time the corona bends down 
toward the oral side, forming the umbrella-shaped stage of the larva. The vesti- 
bule (fig. 2 H v) is then formed by the bending down of the edge of the uvrbrella 
and fusion with the expanded base, forming thus a circular cavity. The walls 
of the vestibule break up and the cavity merges into the general cavity of the 
larval interior. The retractile disk and the basal plate of the sucker are now the 
only remaining larval structures. The former gives origin to the basal part of 
the Avail of the primary zooecium and the latter, after becoming invaginated, 
develops the ectoderm and encloderm of the primary zooid. Besides this sac the 
interior of the larva now contains only a mass of undifferentiated tissue derived 
from the various structures that have disintegrated. The outer wall of this 
tissue forms the wall of the primary zooecium and most of the internal mass forms 
a brown body, although a small part seems to form the mesoderm of the zooid. A 
pouch or diverticulum of the sac forms the rudimentary stomach and intestine 
and a second diverticulum, the esophagus. These fuse and result in the con- 
tinuous alimentary canal. In this process of development the brown body remains 
close to the stomach and is gradually absorbed. After the rudimentary alimentary 
canal has been formed, the Avails of a space in the upper part of the primitive sac 
cavity become changed into the tentacle sheath, on the base of which appear the 
tentacles and lophophore. The nerve ganglion is the result of an invagination of 
the ectoderm in the region between the mouth and anus. 
The primary zooid resulting from the metamorphosis of a larva is the ancestor 
of the entire zoarium and is appropriately called the ancestrula. By a process of 
asexual reproduction (repeated budding) the entire branching structure with its 
numerous zooecia results. Although the studies are still imperfect, it is undoubt- 
edly a fact that each family of bryozoa is characterized by its own particular form 
of larva. This process of budding is discussed in more detail under the general 
considerations of the Cheilostomata below. 
Some zooecia of the adult colony contain no polypide, but in its place is a 
brown body similar to the one described above in the primary zooecium. This 
brown body is the remains of a polypide that has undergone degeneration. In such 
cases the alimentary canal with the lophophore and tentacles have become com- 
pletely absorbed. Such degenerated polypides may become regenerated with a 
redevelopment of the organs and reabsorption of the brown body. 
