iNTuomrnnv. cxxv 



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 are universally tubular ; the 

 polypide is without complexity of structure and has a 

 small number of tentacles ; all appendicular 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 Bryozoaires." 



iii. Ctenostoniata. 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 setae 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 

 setae, 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- 

 l. The Ctenostoniata are also characterized by their 

 horny, or membranous, or membrano-gelatinous tests; 

 they are never calcareous. Marsupia and appeudicular 

 organs are wanting. 



Khlers has criticised the constitution of this division : 

 he considers that the setose operculum is not suffici- 



