482 



POLYZOA 



and may be either prominent or more or less concealed by the 

 growth of adjacent parts of the zooecia. The presence of ovicells 

 of this description is perfectly distinctive of the Cheilostomata. 



Avicularia and Vibracula. Most singular of the external 

 appendages of the Cheilostomata are the extraordinary "avicu- 

 laria" and "vibracula " of some genera. 1 By the comparison of a 

 carefully selected series of genera, it has been established that the 

 avicularium is a special modification of a zooecium. One of its 

 least modified forms is found in Flustra foliacea (Fig. 232), 



Fig. 239. Forms of avicularia. A, Cellaria jistulosa L., Plymouth, x 43; a.z, avicu- 

 larian zooecium, with closed mandible ; o, operculum of zooecium : B, Schizojwrella 

 unicornis Johnst., Scilly Is., x 43 ; zooecium hearing two avicularia ; m, opened 

 mandible 6f avicularium ; s, sinus of orifice : C, zooecium of Smittia landsborovii 

 Johnst., Plymouth, x 43 ; the operculum is seen at the bottom of a depression 

 surrounded by a thin collar or "peristome," in an emargination of which is seen 

 an avicularian zooecium (a.z) ; m, mandible (opened) ; p, pores ; t, tooth. 



where the avicularia (a) are small zooecia with a conspicuously 

 large operculum (" mandible "). Avicularia of a similar type 

 occur in Cellaria (Fig. 239, A), Schizotheca, etc., the avicularium 

 occupying the place of an ordinary zooecium. These are the 

 " vicarious " avicularia of Mr. Busk. 2 



In the next stage (Figs. 239, B, 256, B) the avicularian 

 zooecium is further reduced ; it has in most cases lost its place 

 in the series of individuals, and is found instead seated on some 

 part of an ordinary zooecium (" adventitious " avicularia). The 

 avicularium now consists of a much reduced zooecium, bearing 

 the well-developed operculum or mandible. 



1 See Hincks, Brit. Mar. Polyzoa, i. p. lxiv. 

 tlie British Museum, part ii. 1854, p. 103. 



2 J. Linn. Soc. xv. 1881, p. 359. 



and Busk, Cat. of Marine Polyzoa in 



