709 
4. On the Life History of Eutima, and on radial and !bilateral sym- 
metry in Hydroids. 
By W. K. Briooks, Johns Hopkins Univ. Baltimore. 
eingeg. 27. October 1884. 
Claus has given an account (Arbeiten IV, 2. 1.) of the mature 
hydra of a species which is very similar to Eutima mira (McCr.) and 
he has also described the formation of the medusa buds, and the 
metamorphosis of the young medusa. Although he reared the hydroids 
from the egg he neglected to study the early stages, and his account, 
which contains all that has been published on the history of the group, 
is therefore very incomplete. 
I have reared the hydroid of Eutima mira from the egg, and am 
therefore able to supplement Claus’ account, by a history of the em- 
bryology of the planula, and an account of the young hydroid. 
The early stages present many interesting points ‘which have 
never been described, although more careful observation will un- 
doubtedly result in their discovery in other hydroids. 
The pear-shaped planula is so transparent that the internal 
changes can be studied in the living animal, and it is possible to 
actually witness the delamination of the entoderm from the inner ends 
of the ectoderm cells. This takes place most rapidly at the small end, 
but entoderm cells are formed over the whole inner surface, and they 
arrange themselves in a single layer one cell thick, around a central 
digestive cavity, which soon becomes obliterated. 
After the entoderm is formed, there is a very interesting change, 
which has, as far as I am aware, never been described. The small end 
of the planula becomes elongated and then the entoderm is invagi- 
nated, giving to the embryo almost exactly the appearance of an echi- 
noderm gastrula. The resemblance is so complete that, if the formation 
of the entoderm and digestive cavity had not been witnessed the embryo 
might easily be mistaken for an invaginate gastrula, but the later 
history shows that the invagination has nothing to do with the forma- 
tion of the digestive tract, but is a gland for furnishing the cement 
by which the planula is to fasten itself. 
It is at first at the small end of the body, but as one lip of the 
orifice grows faster than the other, it is soon pushed into one side, so 
that the planula is no longer radially but bilaterally symmetrical, with 
a dorsal and a ventral surface. 
