CXXX INTRODUCTION. 



by Hassall) in the orifice^ which exhibits a series of well- 

 marked modifications^ and has in some cases a develop- 

 mental history, which affords the most valuable, because 

 the most significant, characters. 



In the Escharine* group it seems to me that the 

 families and genera should be based almost wholly on the 

 zooecial character ; but I am certainly not prepared to 

 hold that other structural elements should never be taken 

 into account. The Flustrida, which seem to constitute 

 a most natural group, have a true Membraniporidan cell, 

 and hold their separate place by virtue of their corneous 

 and foliaceous zoaria. It certainly seems desirable that 

 this marked zoarial modification should be distinguished 

 by a separate name. Probably, however, it might be well 

 to include this group and the MembraniporidcB in a single 

 family, based on the unity of the cell. In such a tribe as 

 the Polyzoa there must undoubtedly be some flexibility in 

 our system. 



The principal systematic changes will be found in the 

 Escharine division, where also the chief difficulties have 

 been encountered. Many of the generic groups (such as 

 Membraniporella, Cribrili?ia, Porina, Smittia, Lepralia, 

 Mucronella, &c.) will, I think, commend themselves at 

 once to the student. Some are less strongly marked, 

 but not, I believe, less natural. A few have more doubt- 

 ful claims, and may require revision in the light of fuller 

 research t- In all cases I have taken into account such 

 foreign species as were available in studying our British 

 forms. 



* I use this term as defined by Smitt, to include the old Lepralia, Es- 

 chara, and kindred genera. 



t In a few cases I have not had a suilleiently large number of specimens to 

 allow of the complete study of the form in its rarious stages. 



