THE AXIAL GRADIENTS IN HYDROZOA. 417 



The result is readily understandable when it is recalled that in 

 the regeneration of Tubnlaria, the new hydranth is laid down in 

 the old coenosarc. The new hydranth will then be necessarily 

 broader the stouter the stem. 



It will be perceived that the differences in dimensions of 

 regenerated hydranths on stems of different diameter are in no 

 wise proportional to the diameters. In Table IV., the diameters 

 differ by 40 per cent, on the average, while the differences in 

 dimensions of the regenerated hydranths average 10 to 20 per 

 cent. Further, the more slender stems regenerate in a shorter 

 length of time. 



7. Relation of the Size of the Regenerated Oral Hydranth to the 

 Level of the Stem. In determining this matter it is necessary 

 that the diameter of the apical ends of the pieces to be compared 

 be the same, for, as shown above, diameter affects the dimensions 

 of the regenerated hydranth. It is a little difficult to obtain 

 pieces from different levels of the same apical diameter since in 

 Tubularia the stem generally tapers towards the base. How- 

 ever, it is occasionally possible to find stretches of stem of ap- 

 proximately the same diameter throughout or even some which 

 increase in diameter proximally. Only such have been used in 

 making the comparison. From such stems apical and basal 

 pieces of equal length were cut and after regeneration the di- 

 mensions of the regenerated oral hydranths determined. Some 

 data of this kind, obtained at both Woods Hole and Swan's 

 Island are given in Table V. 



It is obvious to the eye and measurements also demonstrate 

 that in pieces of the same diameter and length but taken from 

 different levels, the dimensions of the regenerated oral hydranth 

 are nearly always greater on the apical than on the basal piece. 

 Level is thus the most important factor in determining the 

 dimensions of regenerated oral hydranths. These size differ- 

 ences of oral hydranths also of course appear in pieces cut from 

 the usual type of stem, where the diameter of the basal piece is 

 smaller than that of the apical piece; and are too great to be 

 accounted for merely on the differences in diameter. 



