POLYZOA OE BRYOZOA. 417 



In order, then, to arrive at a clear conception of the struc- 

 ture of a Polyzoon, we have simply to imagine that such a 

 polypide as above described should have the power of repeat- 

 ing itself by gemmation, " thus producing one or more pre- 

 cisely similar systems, holding a definite position relatively 

 to one another, while all continue organically united." 



The only element of the Polyzoa with which the palaeontol- 

 ogist is concerned is the external investment of the colony 

 the " ccencecium " or " polyzoarium." This is formed by the 

 combined ectocysts of the various polypides, and it varies 

 greatly both in form and actual composition. In form, it 

 may be plant-like, rooted at one point, and rising into folia- 

 ceous expansions or arborescent growths ; or it may spread 

 over some foreign object as a continuous crust. In consist- 

 ence, it may be fleshy, horny, sub-calcareous, or completely 

 calcareous ; the simply fleshy forms, as a matter of course, 

 never occurring in a fossil condition. 



The shape of the " cell," formed by the ectocyst or outer 

 wall of each polypide, varies considerably, and important 

 distinctions may be drawn from 

 this character alone. In one 

 large group of the Polyzoa 

 the Cheilostomata the mouth 

 of the cell is never quite ter- 

 minal in position, but is always 

 placed upon the front of the 

 cell, generally close to one end 

 (fig. 261); whilst the diameter 



Of the mOUth is leSS than the Fi S- 261. MembraniporaOceatii, show- 

 .. . ing the sub-terminal mouths of the cells, 



diameter of the cell. In most upper cretaceous. 

 of these forms, also, the mouth 



of the cell is provided with a movable lid or shutter, by 

 which it can be closed when the animal is retracted within 

 it. In another great group the Cyclostomata the cells 

 are tubular in form, and the mouth is terminal in position, 

 whilst its diameter usually equals that of the cell. In these 

 forms, also, there is no special apparatus for the closure of 

 the mouth of the cell. 



The surface of the cell may be " either smooth and entire, 

 VOL. I. 2 D 



