BROOKS AND RITTENHOUSE: ON TURRITOPSIS. 
449 
is not formed; consequently a true blastula does not exist in the 
development of Turritopsis. In this respect it differs very markedly 
from Stomotoca and the majority of hydromedusae whose develop¬ 
ment has been studied, in which a definite blastocoele is formed that 
becomes filled finally with the migrating endoderm cells. When the 
developing egg is about six to eight hours old, the very irregular shape 
which the segmenting mass has assumed, becomes less marked. 
Gradually the cells become rearranged; the lobes and processes which 
previously were so conspicuous are now drawn into the main mass 
of cells, and the egg is transformed into an oval embryo. This pro¬ 
cess of rounding up lasts from two to four hours. The cells of the 
embryo now develop cilia, and the larva begins to move. At first 
the movements are feeble, but soon the larva is able to leave the bottom 
of the aquarium and swim free in the water. Eggs that are laid at 
from 5 to 6 o’clock in the morning develop to the free-swimming stage 
by 4 in the afternoon. The larva swims with its broad end forward, 
and has a spiral or cockscrew motion, which propels it onward. This 
method of swimming is common to hydroid larvae. When the em¬ 
bryo reaches this stage the cells become very numerous and small. 
Before the cilia are developed and movement begins, it resembles an 
unsegmented egg very much, except that instead of being spherical 
it is now oval. In size it is about the same as the unsegmented egg, 
if anything rather smaller. The decrease in size must be accounted 
for by the fact that some of the yolk has been digested; and the larva 
evidently has not yet acquired any means of receiving food from the 
external world. 
The larva remains in this oval condition for some hours, after which 
it elongates to form a typical planula. When the embryo is twenty- 
four hours old it lengthens out and becomes more slender and assumes 
the general appearance shown in figure 32 (pi. 33). As it becomes 
older it grows still longer. Figure 33 (pi. 33) shows a larva of thirty 
hours. It has now the power of contraction, and is sensitive to stimuli. 
When the cilia are first developed and for some time during the oval 
condition of the larva, it swims near the bottom of the aquarium. 
As it grows older and elongates it rises in the water and swims at or 
near the surface. The length of time during which the embryo re¬ 
mains in the free-swimming planula stage is variable; but, as a rule, 
by the time it is about forty-eight hours old, it begins to sink toward 
the bottom of the aquarium, and to swim less rapidly. After the spiral 
