No. 401.] DEVELOPMENT OF PENNARIA TIARELLA. 391 
part for the higher coloration exhibited by the latter, and might 
naturally occasion the more rapid development of the colonies, 
as well as the greater activity of the medusz, since their expo- 
sure at low tide and always near the surface would give them 
the advantage of the naturally higher temperature of the sur- 
face water. May it not be probable that this condition is an 
adaptive one for the more rapid multiplication of the sexual 
persons which, by their greater activity, secure a greater dis- 
tribution ? For it must be noted that the medusz of the deep- 
water forms are much less active, in many cases never becoming 
free at all, and when becoming so rarely swimming actively. 
During one entire season I failed to detect this feature which 
I have since found to be a very common one, At one time I 
was inclined to suspect that, as McCrady had suggested (57), 
the ova might have developed within the bell of the medusz 
and emerged as planule, for, in more than one collection 
brought in during the early morning, numbers of planulze were 
found among the fucus-bearing hydroids, and among the stems 
of the colonies themselves, while the yet living medusz were 
found still attached to the hydranths. McCrady’s observations 
may have been due in part to the fact that in many cases these 
deeper water medusz do not seem to discharge the ova freely, 
and, since the free exposure of the ova within the bell of the 
medusa insures usually their fertilization, so, as a matter of 
course, the development follows without interruption. It is 
more probable, however, that his observations were due to a 
misinterpretation of the singular phenomena of segmentation, 
to be described later. 
After the discharge of the ova the medusze, when free, con- 
tinue to swim about actively for a time, but soon begin to show 
signs of decline and rarely live beyond a few hours, sometimes 
twelve or even twenty-four, though A. Agassiz has recorded 
their living in confinement for several weeks. My own obser- 
vations, however, show no confirmation of this. They rather 
confirm the observations of McCrady, who remarks upon the 
“apathetic condition of the ipie. following the expulsion of 
the planules." 
In common with most hydroids Pennaria is distinctively a 
