TUBICOLAR ANNELIDA. 
11 
of the tubes and of the apertures become angular, and they present the aspect 
of a colony of Favosites. In other well-preserved groups, the cells, in a greater 
or less degree, retain their individuality, presenting a tubular appearance and 
circular or sub-circular apertures. These conditions are illustrated in figures 
18-23. 
As a general feature these tubes (in the Lower Silurian forms), whether singly 
or in larger or smaller groups, usually become more distinctly and extremely 
annulated as they recede from the initial point, while the longitudinal striae 
become more distinct. It sometimes happens, however, that at an earlier stage 
of growth (figs. 12-14 of plate cxv), but often at a later period, the walls 
become thickened and the annulations are not developed, or they become obso¬ 
lescent or even quite obsolete from this thickening of the tissue. In this con¬ 
dition they present an undulating surface with obscure or indistinct annulations, 
as in figure 27, plate cxv, which may be partially due to erosion. The condi¬ 
tion of surface shown in figures 12-14 is seen in its more extreme degree in 
figure 15; a longitudinal section of which is shown in figure 16, plate cxv. 
We observe in this example the unequal development of the intermural 
cellular tissue which begins at a much earlier stage in some individuals than 
in others. 
When these tubular bodies, in their advanced growth, are cut transversely or 
longitudinally the wall is usually found to consist not of a single lamina, or of 
cohering parallel laminae, but being more or less of a vesicular tissue, which in 
some examples is scarcely, or even not at all developed, while in the majority 
of individuals this texture is a conspicuous feature. This tissue is rarely, if 
ever, observed in the extremely young state. It usually, but not always, in¬ 
creases with age, and upon its development depends the thickening of the walls. 
In some specimens, of apparently adult growth, the vesicular structure is not 
developed, and the inner and outer walls are in contact or separated from each 
other by simple uninterrupted space. The varying degrees of development in 
this intermural tissue are shown in the longitudinal sections, figures 32-39 of 
plate cxv; some of which are from single and separate individuals, and others 
from groups, like those represented in figures 7-13. Figures 15 and 16 illustrate 
