10 BULLETIN OF THE 



spending 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 Phylactoleemata. 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 

 aci'oss in Figure 19 (Plate III.) 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 mesenchymatons 

 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 III. 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 maci'opus also. Its function, as has been fre- 



