390 THE AMERICAN NATURALIST. (VoL. XXXIV. 
III. NATURAL History. 
Pennaria tiarella is one of the most abundant and beautiful 
hydroids to be found in the waters of the northeastern Atlan- 
tic coast, occurring abundantly in tide pools, upon piles of 
docks, fucus, etc. Its generic name indicates one of its most 
conspicuous features, namely, the feather-like form of the main 
stem and its branches, which spring laterally and alternately 
from either side. When growing in dense tufts or colonies 
this feature is often obscured in a measure or, in some cases, 
even lacking entirely. In size it varies greatly as found under 
varying conditions of environment. Under the best conditions 
it may have a height of six inches or even more, while under 
circumstances less favorable it may scarcely have a height of 
more than two inches. 
In this connection may be noted a rather interesting and, so 
far as I know, an unrecorded peculiarity of habitat, namely, its 
occurrence during the summer under apparently two conditions, 
the one appearing considerably earlier and finding a habitat, as 
already indicated, upon rockweed, piles, etc., the other occur- 
ring later and in great abundance upon eelgrass. The latter 
form rarely attains the larger size given, but it matures with 
much greater rapidity and has apparently a much briefer period 
of activity, hardly covering more than about four or five weeks. 
It is further distinguished by a higher coloration of the colo- 
nies and the meduse. Again, the medusz free themselves 
with much greater frequency and ease, and swim much more 
actively. The ova of the two forms likewise show the same 
difference of coloration, those of the latter being a brighter 
orange and much more conspicuous, while those of the former 
are of a creamy white, with the slightest tint of dull pink. 
Morphologically these forms exhibit no constantly distinctive 
differences. The first, from its habit in usually deeper water, 
and thus constantly submerged condition, does not exhibit that 
marked feather-like bilateralism common in the eelgrass form, 
which, from the fact that it often floats upon the surface and, 
at low tide, often lies quite exposed, would come naturally to 
assume the bilateral form. This fact may likewise account in 
