No. 5.] VARIATION AMONG HYDROM EDTS. //;. 225 



Of the further physiological differences one of the most 

 marked is that of the relative activities of the medusae of the 

 afore-mentioned varieties ; those of the surface habit exhibit- 

 ing a much greater degree of activity and other vital phe- 

 nomena. These, as previously pointed out, are extremely 

 active, being liberated from the hydranths promptly upon 

 maturity, swimming with great ease and freedom, and discharg- 

 ing the sexual products with great promptness. On the other 

 hand those of the deep-water habit are passive, or even sluggish, 

 - in many cases the medusae never becoming free from the 

 hydranth, - - discharging the sexual products with much less 

 regularity and ease, and dying very soon after. These medusae 

 are short lived at best and never increase in size after libera- 

 tion from the hydroid. I would suggest the probable correla- 

 tion of some of these features of variation with the degenerative 

 tendency shown in both structural and physiological variations 

 already noted, especially in the atrophy of the chymiferous 

 canals. 



A histological study of the tentaculocysts likewise shows 

 degenerative tendencies, as does also the very rudimentary 

 condition of the tentacles, which are barely distinguished as 

 bud-like protuberances upon the margins of the bell. 



In connection with previous work upon the development of 

 Pennaria, attention was directed to variation in the rate of 

 cleavage and subsequent development. This would seem to 

 be a matter of considerable interest in connection with the 

 fundamental problems of physiological variation. It is well 

 known, of course, that cleavage is a phenomenon subject to con- 

 siderable variation as to rate, due to variable conditions, and 

 to some extent independent of sensible differences of environ- 

 ment. It seems to me, however, that in the case of Pennaria 

 there are presented such marked extremes in this respect that 

 it may well be considered as in some measure correlated with 

 other features of physiological variation. 



The variable rate in the later phases of larval development 

 is also worthy of note. From data obtained during three sum- 

 mers of observation the range of time involved in the larval 

 history varies from about two days up to about two weeks. 



