14 BULLETIN OF THE 
wall muscles and transverse muscles, the latter running from the right 
to the left wall. The fibres of these muscles also break up into branches 
before making their attachments. 
LExcretory Organs. — The chief excretory apparatus in Urnatella lies in 
the calyx below the ectodermal floor of the atrium (Plate III. Fig. 18, 
Plate IV. Fig. 22, nph.). It consists of a pair of tubules which unite 
proximally and open by a single pore into an unpaired cavity, which in 
turn opens into the atrial chamber at about the centre of its floor. 
This unpaired cavity is the one I have proposed to call the cloaca. 
From its opening into the cloaca the unpaired tract of the excretory 
tubules, which may bé designated efferent duct, runs oralwards and down- 
wards, and then divides, the two tubules following the posterior wall of 
the wsophagus. Finally, the tubules turn back upon themselves, run- 
ning outward and towards the rectum. The whole excretory apparatus 
has thus the form of the Greek letter Y. 
Kach tubule ends blindly in a flame cell which bears the characteristic 
cirrus, exactly similar to that found in the stalk (page 6). Figure 22 
(Plate IV.) shows the end of the tubule of the left side. The plane of 
this section was such that it cut the posteriorly reflected region of the 
excretory tubule of the left side throughout all but the middle of its ex- 
tent. At the middle line the tubules of both sides sink below the plane 
of the section, so that it is'the efferent duct which is cut at mph. in the 
median plane. 
One of the youngest individuals in which I have found a nephridium 
is that from which the section Figure 29 (Plate IV.) was drawn. The 
efferent duct (neph.) appears to be composed of two elongated cells 
placed end to end. Running through the midst of these is a poorly 
marked lumen, partly filled by a granular substance. This and one or 
two other similar cases seem to me to support strongly tho view of tho 
intracellular nature of the lumen of the nephridium. 
The evidence derived from the adult condition is less satisfactory, but 
points to the same conclusion. Thus one finds on cross section of the 
tubules that the lumen is not sharply limited like the exterior of the 
tubule. In fact, one sometimes finds delicate threads traversing the lu- 
men (Plate III. Fig. 21, a-c). In one of these sections two nuclei are 
cut across, which in so far militates against my conclusion that tho 
lumen runs inside of single cells placed end to end. But I believe these 
to be the nuclei of two adjacent overlapping cells. 
On account of the evidence just presented, I regard the nephridium 
of Urnatella as having an intracellular lumen and ending blindly in a 
