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 Millepora 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 Milhpora 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 discoverv made by one 

 of us ] ) that the male and female sexual cells of Millepora are borne by free-swimming medusae 

 of a ven- 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, Millepora 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. 



i) S. J. Hickson. Quart Journ. Mier. Sci. 1891 and Proc. Roy. Soc. 1899. 



SIBOGA-EXPED1TIE VIII. 



