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. 



Excretory 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, njih.'). It consists of a pair of tubules which unite 

 proximally and open by a single pore into an unpaired cavity, Avhich 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 be designated efferent duct, mn^ oralwards and down- 

 wards, and then divides, the two tubules following the posterior w^all of 

 the oesophagus. Finally, the tubules turn back upon themselves, run- 

 ning out\vard and towards the rectum. The whole excretory apparatus 

 has thus the form of the Greek letter Y. 



Each 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 eff'erent duct which is cut at njih. 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 

 efl'erent duct (n^ph.) 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 the view of the 

 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-e). In one of these sections two nuclei are 

 cut across, which in so far militates against my conclusion that the 

 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 i-egard the nephridium 

 of Urnatella as having an intracellular lumen and ending blindly in a 



