No. 401.] DEVELOPMENT OF PENNARIA TIARELLA. 393 
called eelgrass forms and those of the deeper water habit. The 
former are liberated usually from about 7 to 8 P.M., followed 
almost immediately by the discharge of the sexual products. 
In the latter form the time of liberation varies from about 10 
to 12 P.M. In each case the male medusz are, as a rule, first 
set free, usually from a few minutes to half an hour. Fertili- 
zation of the eggs occurs at once upon their discharge. 
In Pl. I, Fig. 2, is shown a mature medusa as it appears 
within a few hours of its liberation from the hydroid. At this 
time the nearly mature ova occupy almost the whole interior of 
the bell, often distorting in some measure its form. In no 
case, however, have I observed any such remarkable distortion 
at this stage as that described and figured by Agassiz (65), and 
I am disposed to suspect that his specimen must have been a 
very unusual one, for certainly with ova surrounding the whole 
of the manubrium and compressing it greatly a one-sided dis- 
tortion must be rather difficult to understand. As the ova 
grow about the manubrium they are flattened, disk-shaped bod- 
ies of an outer convex and an inner concave aspect. Approach- 
ing maturity they begin to assume a more or less spherical 
form, as indicated in the figure. Just prior to the release of 
the medusa, and perhaps induced in part by the rhythmic con- 
tractions by which this is effected, they assume a perfectly 
spherical form; the mesenterial membrane becomes more 
closely fitted to each individual egg, and at the moment of 
release the animal has the form exhibited by Pl. I, Fig. 3. Swim- 
ming rapidly about with the characteristic jerking movements 
common to these smaller medusz, the ova are discharged one 
by one as the animal swims, and thus a considerable distribu- 
tion is secured to the ova and the new colonies to arise from 
them. In the growth and maturity of the male medusa essen- 
tially the same phenomena are exhibited. The sperms closely 
pack the whole subumbrella cavity, but as the medusa is about 
to be liberated the mesentery seems to contract, and quickly 
following this, as release is effected, the membrane is ruptured 
and the sperms discharged in immense numbers. 
