THE STYLASTERINA OF THE SIBOGA EXPEDITION 
BY 
SYDNEY J. HICKSON F.R.S. and HELEN M. ENGLAND B.Sc. 
Victoria University of Manchester. 
With three plates. 
INTRODUCTION. 
The association of the genus J/z//epora with the family Stylasteridae in one Order of the 
Hydrozoa called the Hydrocorallina is not justified by recent investigations of their anatomy. 
Apart from the fact that J/eddzpora agrees with the Stylasteridae in forming a continuous 
coenosteum of calcium carbonate and exhibits dimorphism of the zooids, it has no characters 
in common with them. On the other hand in the structure of the nematocysts, the structure 
of the dactylozooids, the general characters of the coenosteum and its relations to the soft parts 
_ Millepora shows no close relationship to the Stylasteridae. Finally the discovery made by one 
of us’) that the male and female sexual cells of A/z//epora are borne by free-swimming medusae 
of a very distinct and definite type necessitates the separation of this genus from the other 
Hydrozoa and the establishment of a new sub-order for its reception. 
It appears to us that to express more accurately the relations of the old group Hydro- 
corallinae, MZz/lepora should be placed in an Order Milleporina situated in our system next to 
the Hydromedusae, whilst the family Stylasteridae should be placed in an Order Stylasterina 
situated near to the Trachymedusae. 
1) S. J. Hickson. Quart Journ. Micr. Sci. 1891 and Proc. Roy. Soc. 1899. 
SIBOGA-EXPEDITIE VIII. I 
