POLYZOA. 



607 



based upon this character. Thus, in the so-called " Cyclostomatous " 

 Polyzoa the cells are tubular in form (fig. 451, a), and the aperture 

 is terminal in position, and is approximately equal to the cell itself 

 in diameter, while there is no special movable apparatus for its 

 closure. On the other hand, in the so-called " Cheilostomatous " 

 Polyzoa (fig. 451, b) the mouth of the cell is never quite terminal 

 in position, but is always placed upon the front of the cell, generally 

 close to the anterior end, while its diameter is less than that of the 

 cell, and it is provided with a movable opercular valve. 



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

 ous or granulous ; perforated with minute pores, or cribriform with 

 larger openings ; reticulate or ribbed, &c, — all of which conditions, 

 with certain precautions, afford excellent diagnostic characters " 

 (Busk). The margins of the mouth of the cell, also, may be 

 " simple or thickened, unarmed or beset with erect ' marginal 

 spines,' which again may be either rigid or articulated at the base, 

 simple or branched/' 



Though the separate zocecia of a Polyzoan colony are usually 

 apparently quite separate and distinct from one another, except by 

 continuity of their external investment, it has been shown that 

 contiguous cells are commonly placed in direct connection with 



* *T— X 



w m 



Fig. 452. — A, Longitudinal section of a few tubes of the recent Tetviysonia stellata, enlarged, 

 showing the porous walls of the cells; B, Central portion of a transverse section of a branch of 

 Cellepora ramulosa (Recent), showing tubes connecting the cavities of adjoining cells, enlarged ; 

 c, Part of a tangential section of the polyzoary of Domopora stellata (Recent), enlarged to show 

 the connecting-tubuli. (Original.) 



one another by what have been called "communication-plates" 

 or "rosette-plates." These are portions of the cell-wall pierced 

 by one or more minute pores which transmit processes of the 

 structure which has been previously described as the " endosarc." 

 In many of the calcareous Polyzoa also, the walls of the cells 

 are pierced by more or less numerous pores, which open into the 

 cavity of the cell (fig. 451). In certain of the Cyclostomatous 

 Polyzoa, as also in some Cheilostomatous forms, where the walls of 

 the cells are of considerable relative thickness, these pores assume 



