CXX1V INTRODUCTION. 



arms, bearing a resemblance to the branchial appendages 

 of the Brachiopoda. 



The Holobranchia form two cardinal groups the Ecto- 

 />rocta, in which the lophophore surrounds the mouth 

 only ; and the Entoprocta, in which it incloses both the 

 orifices of the alimentary canal *. 



The former division embraces the great body of the 

 Polyzoa ; and its marine forms are comprised in a single 

 order, the Gymnolcemata, distinguished by two principal 

 features, the complete abortion of the foot and the cir- 

 cular disposition of the tentacles. 



The Gymnolamata I have ranged under the three well- 

 known suborders founded by Busk on certain structural 

 peculiarities of the cell. 



i. Cheilostomata. These are distinguished by the pre- 

 sence of a movable corneous valve, closing the orifice 

 through which the tentacular corona is protruded ; the 

 operculum is furnished with two sets of muscles, by 

 means of which it is opened and shut. In this division 

 we meet with some special structures which are wanting 

 elsewhere : the ova are commonly matured in distinct 

 marsupia ; appendicular organs (avicularia and vibracula) , 

 which, as we have seen, are modifications of the Cheilo- 

 stome cell, are present in great abundance. The calca- 

 reous test, in a large proportion of the forms, is highly 

 developed ; and there is also considerable variety, and, in 



* Barrels has studied the development of the corona in Alcyonidium 

 (op. tit. p. 257, pi. xvi.), and finds that at a certain stage the arrangement 

 of the tentacles is temporarily the same as that which is permanent in the 

 Entoprocta ; this is followed by a division of the corona into two symme- 

 trical halves, as amongst the Phyltictolamata ; and this gives place to (In- 

 typical conformation of the Ectoprocta. The developmental history, there- 

 fore, gives us the comparative rank of these tliree divisions, and shows the 

 Entoprocta to be the lowest in the scale. 



