No. 6.] AMERICAN MORPHOLOGICAL SOCIETY. 363 



to certain body characters : length, length of bust (to fifth 

 abdominal segment), width and depth of bust, frontal stature of 

 bust (ratio of middle to length), and sagittal stature of bust (ratio 

 of depth to length). In addition the length, width, and stature 

 of a typical organ, the left antenna, were determined. From 

 a tabulation of certain bases of comparison --mean, standard 

 deviation, coefficient of variability - - it appeared : (i) That elimi- 

 nated male pupae were on the whole more variable than the 

 surviving- males, and that the surviving females were far less 

 variable than the dead ones. (2) Only 180 of the 310 living 

 pupae produced perfect moths. The perfect male moths were 

 from pupae, which were far less variable than the others. This 

 condition was reversed in the case of the females, yet the sur- 

 viving females, though more variable than the eliminated ones, 

 were not as variable as the eliminated female pupae. (3) The 

 males of all groups were more variable than the females. 



XXVII. THE ORIGIN OF TENTACLES 

 IN GONIONEMUS. 



H. F. PERKINS. 



SOME interesting data have been secured from the study of 

 the origin of tentacles in Gonionemus, a common Woods Holl 

 hydromedusa. Specimens \ mm. in diameter having from 8 

 to 1 6 tentacles are found in early summer, and in examining 

 these it was seen that there existed a definite relative position 

 of the tentacles and sense organs. 



Two pairs of tentacles, the radial ones, are of equal size. 

 The other tentacles and sense organs are regularly graded 

 from large to small, so that it is possible to determine their 

 order of origin. 



Looking at the marginal ring from below, in a normal 

 medusa, each newly formed tentacle is seen to lie next to a 

 sense organ and to precede it, as the hands of a watch move. 

 Fig. i shows a typical 8-tentacled medusa. Tentacles I and 

 II are radial in position; III follows I in the direction of the 



