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 



