554 POLYZOA. 



middle of the mandible; the contraction of this muscle causes 

 the lower jaw to close upon the upper, and there are two small 

 muscles which bring about the opening. 



There are all stages between such an avicularium as that just 

 described, and a small zooecium with a movable operculum. From 

 Flustra foliacea, Cellaria, etc., in which the avicularium occupies 

 the place of an ordinary zooecium (vicarious avicularia), we pass 

 through stages in which the avicularium, though still a recognizable 

 zooecium, has lost the place of a zooecium in the colony, and is 

 attached to some part of an ordinary zooecium (adventitious) until 

 we reach the highly specialized form just described. 



Vibracula (Fig. 443) are merely avicularia with the beak much 

 elongated, as a long whip-like seta, which has the power of sweeping 



through the water. The function of 

 avicularia is obscure. They have been 

 observed to seize small organisms, e.g. 

 worms, and to hold them until they 

 are dead; the decomposing organic 

 remains may possibly be swept into 

 the mouth by the currents caused by 

 the cilia of the tentacles. 



Finally there are the ovicells of 

 some Cyclostomata (see above, p. 550), 

 which are zooecia with a vestigial 

 polypide.* 



FIG. 443. Sempocellaria ferox (after ^. 



Busk), vi vibracuia. ihe torm ot the cells, and the 



manner in which they are connected 



together, are very different in the different groups, and give rise to a 

 great variety in the form of the colonies. The zooecia are usually 

 completely cut off from one another, though the soft parts are 

 continuous through pores in the ectocysts of adjacent zooecia (rosette 

 plates, communication plates). In the Pliylactolaemata the body- 

 cavities of neighbouring zooecia are in open communication. These 

 two conditions result from the manner of budding : in the first case 

 the bud, which arises as an outgrowth of the body-wall, becomes 

 completely cut off by an ingrowth of body-wall ; while in the second 

 no such partition is formed, so that the body-cavity of the whole 

 colony is a single, continuous space. 



The brown bodies, which are present in most colonies of the Polyzoa, 



* In some Cyclostomes (Lichenopora, etc.) the polypide is fully formed and 

 functional in the young ovicells. 



