10 BULLETIN OF THE 
sponding. region exists and sphincter muscles are found here, it has not 
become so distinctly differentiated from the rest of the body wall as has 
the Randwulst of Phylactolemata. This region may be designated the 
lip of the atrium. 
When the tentacles are expanded, —a secondary condition, as the 
development of the calyx shows, — the lip of the atrium forms a circu- 
lar ridge lying at the base of the tentacles (Plate IV. Fig. 26, spht. atr.). 
When the tentacles are drawn in tightly, the lip of the atrium becomes 
puckered. Two of the folds resulting from this process are shown cut 
across in Figure 19 (Plate ILL.) above the middle of the atrium. 
The tentacles of Urnatella in three cases in which I counted them on 
transverse sections, as well as on the entire animal, numbered twelve ; 
in one case, thirteen. In addition to these numbers, Leidy ('84, p. 10) 
found sixteen (usually) and fourteen. In the specimen with thirteen 
tentacles, the odd one was placed on the anal aspect of the calyx in the 
median plane. It appeared shorter than the others. In one case with 
twelve tentacles, observed fully expanded, the two tentacles of the anal 
aspect lying nearest the median plane appeared shorter than the re- 
maining ones. Leidy does not refer to this point, and his figures 
afford no satisfactory evidence as to the occurrence of this condition 
in his specimens. 
The tentacles are each composed of a cylinder of columnar epithelium 
surrounding a narrow central region which is filled with mesenchymatous 
tissue. In addition, on each of the lateral aspects of the tentacle 
there is a muscle, composed of one to three fibres lying side by side 
(Plate IV. Fig. 27, mu. ret. ta.). The epithelium is ciliated on the lat- 
eral and inner faces of the tentacle. 
The atrium is bounded by the tentacular corona on all sides. The 
floor of the atrium passes into the mouth in the oral region, and rests 
upon the rectum in the aboral region, At the centre there opens into 
it an elongated pocket, the cloaca, The lateral angles of the mouth 
are prolonged aborally, and form two grooves which open into the atrium 
along the lateral margins of the floor (Plate IIT. Fig. 19, sul. atr.). 
These, which may be called the atrial grooves, approach each other and 
become shallower as they pass aborally upon the atrial wall, until they 
disappear in the median line above the rectum. The epithelium lining 
the grooves is ciliated. 
This “atrial groove ” exists also in Pedicellina echinata according to 
Nitsche (69, pp. 21, 22), and, according to Ehlers ('90, pp. 52, 53, 59, 
60), in his Ascopodaria macropus also. Its function, as has been fre- 
