No. 401.] DEVELOPMENT OF PENNARIA TIARELLA. 403 
stem, including the branch. This is in rather striking con- 
trast with those figured in the plates and in the well-known 
annulations of the stems of the Pennaridz. I was for a time 
constrained to wonder whether this might not have been a speci- 
men of some other genus which had accidentally been brought 
in with the collections and had developed along with the Pen- 
naria. Such a supposition is not impossible of course, yet the 
general aspects of the hydroid are so peculiarly pennarian, the 
annulations excepted, that I am rather 
constrained to regard it as simply a 
specimen which exhibits, in an unusual 
degree, a phenomenon which in itself is 
one of the most variable among hydroid 
characters, and that it shows how un- 
reliable must be such a character for 
diagnostic purposes. 
VIII. EXPERIMENTAL. 
1. Darkness. — The liberation of the 
medusæ and the discharge of the sexual 
products upon the approach of darkness : 
suggested this as a possible cause of the Pech 
unusual activity at this time. Accordingly, several experiments 
were made to determine whether such were really the case. 
Colonies of both sexes were collected about 3 p.m. and were 
carefully excluded from light in suitable receptacles, but in no 
case could they be induced to discharge their products or become 
free at an earlier time, though that such conditions might have 
at some time in the life of the race been a factor in determin- 
ing the periodicity of their maturity and release may not be 
improbable. But, as has already been pointed out, the fact 
that the deeper water forms do not become free or discharge 
the sexual products until toward midnight would certainly 
seem to suggest that darkness alone could not be the deter- 
mining factor. 
2. Temperature. — The observation that during specially 
warm weather larger numbers were liberated suggested the 
