MOLLUSCOIDEA — BRYOZOA 1 79 



porous walls and are divided into compartments by numerous 

 transverse partitions (diaphragms) . The lowest portion of each 

 tube is the narrowest, having been built by the individual in 

 its youth ; from this point the tube rapidly enlarges to its maxi- 

 mum, normal diameter. The diaphragms mark its withdrawal 

 from a lower to a higher position. Unlike Bugula, each tube 

 increases in length throughout life. Since, however, the soft parts 

 of the animal increase in size but little, the lower (older), portions 

 of its house-like skeleton must be vacated ; this removal takes 

 place at more or less regular intervals, probably due to a con- 

 traction of the soft parts of the body following the extrusion of 

 the sexual elements, and w^herever the base of the body comes 

 to rest it secretes a calcareous plate, the diaphragm. The tube 

 openings are polygonal or rounded. The surface of the colony 

 is often made irregularly rough by the elongation of groups 

 of individual tubes so as to form small elevations, — monticules 

 (whence the name from Latin monticulus, sl little hill, + porus, 

 a pore). 



1. Sketch (a) outline of entire colony, (b) a view of three 

 zooecia showing both the tubes and their openings, also dia- 

 phragms in one. Label zooecia, diaphragms. 



2. What may probably be the cause of the diaphragms? 



3. How do the zooecia differ in composition from those of 

 Bugula ? 



4. Indicate in one zooecium the position last occupied by 

 the soft portions of the animal. 



Fenestella (Fig. 72). Silurian to Permian. 



Colony usually funnel-shaped, composed of a calcareous 

 net-like frame-work made up of straight radiating branches 

 united by cross bars (whence the name from Latin fenestra, 

 a window, -|- the diminutive ending ella). Upon the upper 

 (inner) side of the radiating branches are two rows of small pits 

 (zooecia), separated by a median keel-like ridge ; each of these pits 

 lodged the soft body portion of an individual bryozoon. The 

 cross bars do not bear pits. In development the large first, 



