488 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Only exceptionally does a nucleus occur in the intermediate zone which has 

 not been drawn into the bases of the papillae ; such a nucleus causes a slight swell- 

 ing in the thin intermediate layer. 



According to the state of contraction the nucleated bases of the papillae some- 

 times project slightly into the underlying hydrocojle wall, and sometimes form 

 an arch bowed outward from it. 



The number of the nuclei assembled in the base of the papillae is not constant. 

 In the central portion of the older tentacles it varies between 20 and 30, the number 

 of the nuclei, as well as the area of the base of the papillae, having increased over 

 what it was in the younger stages. The nuclei are almost always arranged in a 

 circular or oval disk, and differ somewhat among themselves in size as well as 

 in chromatin content. Though most of them are in the resting stage, individual 

 nuclei can always be foimd in course of division. 



From the base of the tentacle there rises a homogeneous glassy cylindrical 

 structure which in extreme cases measures 36 \i. in length, though often it is 

 considerably shortened, and thicker. The tip is expanded, three or four cornered, 

 and from each of the corners there projects a fine hairlike process with sometimes 

 an additional one in the middle. The insertions of these hairs stain with especial 

 facility, and they appear to possess broadened ends sunken into the end of the 

 papilla. The axis is traversed by a fine glistening thread, the outer end of which 

 passes over into the deeply colored end of the median hair, the inner end broaden- 

 ing out, attaching itself to a nucleus of the papilla base, and merging with the 

 plasma of the cell body. In younger stages, when this axial fiber is first formed, 

 it appears to grow outward from the base to the tip. 



In the basal section of the 15 tentacles the papillae gradually become smaller, 

 and finally disappear entirely, the aggregations of nuclei becoming less and less 

 clearly defined. 



On the base of the tentacles in the immediate vicinity of the nerve ring the 

 ectoderm of the inner and lateral walls is always similar and more or less flat- 

 tened. On the contrary, the outer basal wall passes over gradually into the outer 

 body wall of the calyx, into the inner basal ectoderm of the five mouth flaps 

 containing the orals. The true ectodermal epithelium of the tentacles does not 

 extend so far basalward on the outer side as on the inner, for on the former the 

 hydroccele of the tentacle at the base is covered by the lateral portion of the 

 mouth flaps. Thus it comes about that whenever the tentacle bases are turned 

 inward the mouth flaps and the included oral plates necessarily close down. 



Histologically the structure of the ectoderm of the interradial tentacles is 

 somewhat anomalous. The conditions are essentially those found in the large 

 tentacles when they were approximately of the same length. Papillae, smaller 

 than those of the larger tentacles and more or less conical in shape, cover the 

 surfaces toward the larger tentacles of the same parameres, elsewhere occurring 

 only above the fold uniting the two. In the distal portion these papillte are 

 longer than in the basal, where they finally become reduced to small swellings. 

 They are more crowded than on the large tentacles, and the intermediate zones 



