NORTH AMERICAN KARLY 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} provided with motionless sensory cilia, develops at the 

 opposite or aboral pole. 



The larva now escapes from the ovicell 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 zooid. 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 /') is then formed by the bending down of the edge of the umbrella 

 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 wall of the primary zooeeium and the latter, after becoming invaginated, 

 develops the ectoderm and endoderm 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 walls 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 imagination 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 lias 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. 



