226 
ORGANOGRAPHY. 
BOOK I. 
no doubt of the reproductive functions of the contents of what 
is named the theca or capsule, which is a hollow urn-like body, 
containing sporules : it is usually elevated on a stalk, named 
the seta, with a bulbous base, surrounded by leaves of a dif- 
ferent form from the rest, and distinguished by the name of 
perichcetial leaves. If this theca be examined in its youngest 
state, it will be seen to form one of several small sessile ovate 
bodies {pistillidia, Agardh ; prosphi/ses, ^hrhaxt ; adductores, 
Hedwig), enveloped in a membrane tapering upwards into a 
point ; when abortive they are called paraphyses. In process 
of time the most central of these bodies swells, and bursts its 
membranous covering, of which the greatest part is carried 
upwards on its point, while the seta on which the theca is sup- 
ported lengthens. This part, so carried upwards, is named 
the calyptra : if it is torn away equally from its base, so as to 
hang regularly over the theca, it is said to be mitriform ; but 
if it is ruptured on one side by the expansion of the theca, 
which is more frequently the case, it is denominated dimidiate. 
When the calyptra has fallen off or is removed, the theca is 
seen to be closed by a lid terminating in a beak or rostrum : 
this lid is the operculum, and is either deciduous or persistent. 
If the interior of the theca be now investigated, it will be 
found that the centre is occupied by an axis, called the co- 
lumella ; and that the space between the columella and the 
sides of the theca is filled with sporules. The brim of the 
theca is furnished with an elastic external ring, or annulus^ 
and an interior apparatus, called the peristomium : this is 
formed of two distinct membranes, one of which originates 
in the outer coating of the theca, the other in the inner coat; 
hence they are named the outer and inner peristomia. The 
nature of the peristomium is practically determined at the 
period of the maturity of the theca. At this time both mem- 
branes are occasionally obliterated ; but this is an unfrequent 
occurrence : sometimes one membrane only remains, either 
divided into divisions, called teeth, which are always some 
multiple of four, varying from that number as high as eighty, 
or stretching across the orifice of the theca, which is closed up 
by it; this is sometimes named the tympanum. Most fre- 
quently both membranes are present, divided into teeth, from 
