BROAD-LEAVED HORNWRACK. 



237 



zooecium is cut off in the bud by a double fold of the endocyst, which grows 

 inwards from all sides simultaneously, and finally closes in the centre. The 

 dividing wall of ectocyst is formed between the two halves of the fold. 



If a zooecium is situated at the free edge of the object on which the colony 

 grows, or is in contact with some obstacle, its upper surface developes a large 

 cylindrical outgrowth three to four times as high as the zooecium is long, with a 

 thickened ectocyst, in which, however, no calcareous matter is deposited. Such 

 zooecia are termed by Nitsche ' Turret-zooids.' They possess neither tentacles, 

 tentacle-sheath, nor digestive tract. Under normal conditions these organs are 

 developed within the new-formed zooecium by a process perhaps not yet fully 

 explained. Every zooecium remains in connection with its neighbours by means of 

 the ' rosette ' plates, the columnar cells covering them, and the system of funicular 

 cords (supra), to which a nervous function was at one time assigned. 



Flustra foliacea resembles Membranipora membranacea in all essential features. 

 It possesses thirteen to fourteen tentacles ; its tentacle-sheath and digestive tract 

 are long, and much bent and folded in the retracted condition. It possesses Avi- 

 cularia, but they are not much modified in shape from the ordinary zooecium. An 

 Avicularium is a zooid in which the digestive tract is aborted with the tentacles and 

 tentacle-sheath, whilst the rest of the zooecium and the operculum persist. In its 

 most specialised form, e.g. in Bugula (Fig. 10. B.), one of the Cheilostomata, the 

 avicularium is borne upon a moveable peduncle (/.) ; the operculum has become a 

 moveable mandible (m.\ the zooecium a hollow chamber (<r.) bearing the mandible, 

 containing posteriorly a set of divaricator (d.m.) and occlusor (o.m.) muscles, and 

 anteriorly fashioned into a beak (.). Between this highly specialised form and the 

 simple form seen in Flustra all degrees of specialisation may be found. In some 

 Cheilostomata another remarkable form of zooid, shown in Fig. 10. C., occurs. This 

 is the Vibraculum, It possesses a moveable seta (s.\ the homologue of the oper- 

 culum carried on a simple chamber (<r.) containing the muscles (m.) that move the 

 seta. A long tubular appendage originates in many instances from an aperture (a.) 

 in the lower part of the chamber. Transitional forms from such a Vibraculum as 

 this of Scrupocellaria to the Avicularium or zooecium are to be found. The func- 

 tion of the Avicularium is doubtful. In the most specialised forms the surface of 

 the beak turned to the mandible has a small depression, from which project a 

 number of tactile (?) setae with a cellular ganglionic (?) body at their base. The 

 mandible is in constant motion, and the Avicularium has been observed to catch 

 and retain small worms. In the less specialised forms, however, such an action 

 would be impossible. The Vibracula are swept from time to time over the surface 

 of the colony, sometimes with apparently concerted action, as e.g. in Caberea^ and 

 probably remove foreign bodies, &c. They appear to act as locomotor organs for 

 the free colony of the Selenariidae, See Hincks, Marine Polyzoa, i. Introduction, 

 p. Ixiv. to p. Ixxxiii. 



Flustra also possesses Ooecia, or marsupial chambers, into which the ovum 

 passes and undergoes its development. These Ooecia are produced at the distal 

 end of the zooid. For their structure and development, see Nitsche, Z. W. Z. xx 

 1870, p. 3, pi. i. figs. 10-13, and Vigelius, Biol. Centralbl. iii. 1883-84, p. 710. 

 The zooecial character of these organs is doubtful. In some Polyzoa the zooecium 

 is modified into a Stem-cell or a generative zooid (=Gonoecium or Gonocyst). The 



