xlviii 
THE VOYAGE OF H.M.S. CHALLENGER. 
The embryo now extends itself and assumes an ellipsoidal form. On one pole the 
walls grow thinner and soon present a stellate cleft which becomes the mouth. 
Simultaneously with the formation of the mouth the tentacles show themselves as short, 
blunt, conical outbulgings of the body walls. 
The embryo commonly remains two or three days within the covering of the inner 
shell. This, however, gradually softens and becomes changed into a tenacious adhesive 
mucus, which is finally dissolved in the water, and the a nim al becomes free with all the 
essential characters of the adult. 
It will be thus seen that in Hydra no gonophore is formed as an independent 
zooid, and that the generative elements not only originate in the walls of the body but 
are retained there up to the period of their liberation in the surrounding water. It will 
also be apparent that in Hydra development takes place without the post-embryonic 
period of its life presenting anything like a true metamorphosis, and that there is 
here nothing which can be regarded as a proper larva stage. In this respect Tubu- 
laria agrees most nearly with Hydra, but here the Actinula is a true larva differing 
from the adult not only in its free mode of life but in its form, though it has no obvious 
organs which are not also present in a more or less modified shape in the adult. Myrio- 
thela differs still further in having a more distinctly pronounced larval stage, and in 
undergoing a well-marked metamorphosis, the Actinula of Myriothela being provided in 
a very characteristic way with transitory organs which entirely disappear during the 
further course of the development. 
