POLYZOA. 193 



the simply fleshy forms, as a matter of course, never occurring 

 in a fossil condition. 



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

 wall of each polypide, varies considerably, and important dis- 

 tinctions may be drawn from this character alone. In one 

 large group of the Polyzoa the Cheilostomata the mouth of 

 the cell is never quite terminal 

 in position, but is always placed 

 upon the front of the cell, gene- 

 rally close to one end (fig. 132) ; 

 whilst the diameter of the mouth 

 is less than the diameter of the 

 cell. In most 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 Fig. ^-^.Escharina Oceani, show- 

 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, 

 spinous or granulous ; perforated with minute pores, or cribri- 

 form 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." 



There still remain three structures which are present in many 

 forms, and especially in the Cheilostomata, which require some 

 notice. The structures in question are known as the "ovicell," 

 the " avicularia," and the " vibracula." 



The " ovicell " is a structure especially characteristic of the 

 Cheilostomatous Polyzoa; but its presence is not universal, and 

 when present it may be inconspicuous. Its general form is that 

 of "a more or less rounded eminence situated above or behind 

 the cell. . . . The cavity of the organ is continuous with 

 the perivisceral space, through a passage situated at the upper 

 and back part of the cell, and through which it would appear 

 the ova are conveyed as into a sort of marsupial pouch. This 

 organ is wanting in the Cyclostomata, in which its functions are 



