1894,] Hydroid: Cold Spring Harbor, Long Island. 81 
final separation of the medusa. The inner layer of the ecto- 
derm continues to invaginate, making the ectodermal lining of 
the bell, and giving rise to the tentacles and velum. Cf. figs. 
9 and 10. 
From a glance at figs. 11 and 12, it will be seen that the 
tentacles are relatively very long, but in mature specimens 
they are much longer than represented. Their disposition 
during the period of development is quite interesting. It is 
measurably indicated in fig. 9. They are from the first turned 
inward and soon enter the interior of the bell, being nicely 
folded and filling the entire cavity. This habit is not 
abandoned immediately on the separation of the medusa. In 
many cases under observation the medusa, when suddenly dis- 
turbed, would at once contract the tentacles, and, seeking the 
mouth of the bell, dispose them within, and at the same time 
assume an almost spherical form and thus remain for some time 
or till the disturbance ceased. 
The development of the radial and circumferential gastric 
canals proceeded, pari passu, with the growth of the medusa. 
This was very easily demonstrated by the active circulation 
which was observed extending to the very tips of the tentacles 
and over the middle of the bell to unite with the circumferen- 
tial canal at the optic spots. No special histological investiga- 
