INTRODUCTION. C'XXV 



many cases, a good deal of specialization in the structure 

 of the polypide. 



ii. Cyclostomata. — These, which are the earliest extant 

 forms, are distinguished by the simple, inoperculate 

 orifice of the zooecium. 



Simplicity, indeed, is in the highest degree character- 

 istic of the group : the cells ai'e universally tubular ; the 

 polypide is without complexity of structure and has a 

 small number of tentacles ; all apj^endicular organs are 

 wanting. There is also a remarkable uniformity in the 

 embryology; and the larvae of the leading families are 

 identical. Barrois is right in describing it as '^le groupe 

 le plus naturel et le mieux circonscrit de la classe entiere 

 des Brvozoaires." 



iii. Ctenostomata. — The leading characteristic of this 

 division is the curious opercular mechanism of the cell. 

 The upper portion of the cell-wall is of slighter material 

 than the rest, and terminates above in a number of deli- 

 cate setse held together by a thin transparent membrane. 

 (See Woodcut, 35 ec' and op, p. 562.) When the poly- 

 pide retreats, the whole of this anterior portion is drawn 

 in by means of a special apparatus of muscles, and the 

 setse, being brought closely together, form an operculum 

 (or protective covering) above the tentacles. This ar- 

 rangement secures the freest play for the corona when 

 exserted, and perfect security when the polypide with- 

 draws. The Ctenostomata are also characterized by their 

 horny, or membranous, or membrano-gelatinous tests ; 

 they are never calcareous. Marsupia and appendicular 

 organs are wanting. 



Ehlers has criticised the constitution of this division : 

 he considers that the setose operculum is not sufliici- 



