392 THE AMERICAN NATURALIST. | [Vor. XXXIV. 
summer organism, at least in its actively vegetative condition. 
I have not found records of its occurrence in flourishing condi- 
tion later than November, and during several years of observa- 
tion have not taken it in mature form earlier than June, though 
its seasonal variation may be considerable. After its more 
active period there is an evident decline in vigor, and later a 
degeneration of the polyps and hydrocaulus, and a recession of 
the ccenosarc within the stoloniferous hydrorhiza, followed by 
a period, more or less prolonged, of quiescence. This is chiefly 
a seasonal peculiarity, induced doubtless by coincident changes 
of temperature, food, etc. It may occasionally, however, be 
induced by conditions purely local and temporary. Colonies 
now and then show an evident decline for a period of varying 
extent and then may revive and regain their normal activity 
within a few weeks. In these cases no discernible cause is 
evident, and hence they have been designated as purely local 
and temporary. 
Pennaria exhibits in a very marked way the phenomenon of 
“alternation of generations." The nonsexual, or vegetative, 
phase comprises the splendid hydroid features whose natural 
history has now been outlined. Its sexual, or medusa, phase 
presents an organism which Agassiz (65) has characterized as 
“one of the most remarkable of our naked-eyed medusa." In 
form it is elongate-oval, slightly smaller at the oral end. In 
Pl. I, Figs. 2 and 3, are shown adult medusz with developing 
and mature eggs. Its size varies slightly, but averages about 
I.5 mm. in length, and .8 mm. in short diameter. 
In its natural history the medusa exhibits some very interest- 
ing features. Some of these have already been pointed out, 
such as the variation in coloration, activity, etc. Smallwood 
(99) has observed in working out its development evidences 
of degenerative conditions which harmonize with some of the 
observations previously cited, and with similar conditions 
among not a few other of the Hydromedusz, e.g., Clava, 
Hydractinia, etc. 
The medusz mature with comparative rapidity and show a 
rather marked periodicity as to the time of liberation. And 
here again there is a rather sharp difference between the so- 
