418 MOLLUSC A. 



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 re- 

 quire 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 uni- 

 versal, 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 Cydostomata, in which its functions are apparently sup- 

 plied by a dilatation of the body of the cell itself." (Busk.) 



The "avicularia" and "vibracula" are peculiar appendages 

 of the ectocyst, supposed to be weapons of offence and de- 

 fence, or to subserve some unknown function in the economy 

 of the colony, and believed by Prof. Huxley to be peculiarly 

 modified polypides. The avicularia, or " bird's - head pro- 

 cesses," differ a good deal in shape, but consist essentially of 

 a "movable mandible and a cup furnished with a horny 

 beak, with which the point of the mandible is capable of 

 being brought into apposition " (Busk). In shape they are 

 often closely similar to the head of a bird (fig. 260, 3), and 

 they perform a peculiar snapping movement, which is con- 

 tinued long after the apparent death of the colony. In 

 many respects the avicularia are comparable with the 

 " pedicellarise " of the Sea-urchins and Star-fishes. In the 

 " vibracula," the place of the mandible of the avicularium 

 is taken by a bristle or seta, which is capable of extensive 

 movement. 



