NORTH AMERICAN EARLY TERTIARY BRYOZOA. 
637 
are parallel to each other and their length is unlimited. They give rise easily 
to adventitious tubes. 
In transverse section ad such tubes appear of the same size at the center 
as at the periphery; the smaller tidies dispersed irregularly between the large ones 
are the young tubes (fig. 197 B). 
In longitudinal section the interzooecial spaces are generally vesicular through- 
out their course in the massive forms (fig. 197 A), but only at their extremity 
in the ramose forms (fig. 197 C). However there are numerous exceptions to 
this. These vesicles are sometimes quite large (Fig. 197 D) ; we are ignorant 
of their function. There is no zone of growth visible, since the tubes have the 
same diameter. 
The ancestrular tubes are short and not strictly parallel to each other. They 
rise more or less in a spiral around the ancfestrula and form the nuclear region, 
with nonorientecl tubes (^immature region of the Trepostomata) (fig. 197 A). 
Club-shaped tubes or typical Cyclostomata, — These increase regularly iir diame- 
ter during the greater part of their length. They are slightly oblique to each 
other and always have a part at the exterior of the zoarium. Their length is 
limited. They rarely give rise to adventitious tubes. They become cylindrical 
when near the zone of growth or when they bend toward the exterior, or finally 
when they curve at a right angle with production of adventitious tubes. The 
last case is without exception. 
In transverse section these tubes appear smaller at the center than at the cir- 
cumference (fig. 198 A. B). In longitudinal section they are separated generally 
by a linear space, rarely vesicular (fig. 198 C). 
The zone of growth is the ensemble of the small orifices of the incompletely 
developed tubes; it is quite variable in form and size (fig. 198 D. E). The 
lamella on which the new tubes are developed is called the basal lamella (=germinal 
lamella of D’Orbigny). 
Tubes with facets (Metopoponna) . — These are expanded tubes in which the 
peristome is closed by a facet (D’Orbigny) or zooecial area (Levinsen). This 
area (fig. 199) is perforated by an orbicular or semilunar orifice destined for 
the passage of the tentacles. The orifice is often closed by a sort of calcareous 
operculum. The zooecial walls are thickened at their extremity. 
This form of tubes has completely disappeared from the recent seas; they 
correspond therefore to some anatomical features rather difficult to reestablish. 
Such a form exists in many families: Melicerititidae, Lobosoeciidae, Plagioeeiidae, 
and other families. 
Tubes with rhomboid ad orifice. — These are cylindrical or funnel-shaped. Their 
aperture is oblique to the zooecial axis; it is not orbicular, but is elongated and 
rhomboidal (fig. 200). The bundles are caused by the reunion of many tubes and 
form zoarial and lateral expansions symmetrically arranged, called pinnules. They 
comprise many genera of the family Cytisidae. This form of tubes has disappeared 
from the recent seas. 
