444 Report of the State Geologist. 



opening, marking the point from which a long tubular appendage 

 (or radical fiber) originates. 



" The vibraculum, as already mentioned, is of comparatively 

 rare occurrence. In its most highly specialized forms it is placed 

 on the dorsal surface of the zooecium, and the movable seta 

 (which, when at rest, is laid back upon the chamber) is swung 

 around at intervals to the front of the cell, sweeping slowly over 

 the surface as if to remove all noxious matter, and then returns 

 to its original position. This movement goes on uninterruptedly 

 during the lifetime of the colony, and there can be no doubt 

 that its object is to scare away dangerous intruders or accumula- 

 tions of refuse from the neighborhood of the orifice. We have 

 no difSculty in recognizing the equivalent of the avicularian 

 mandible and the operculum of the cell in the setae. The mouth 

 is here modified in the same sense as the rest of the structure ; 

 the raised 'beak' is absent, being no longer useful, but the mar- 

 gin is carried out above into two prominent points, just within 

 which the bristle is articulated, clear of all hindrances, and so as 

 to possess the utmost freedom of movement. 



" The homology of the parts becomes more evident when we 

 study the transitional forms. We meet with a developmental 

 stage (corresponding to the primary avicularium) in which the 

 vibraculum is developed on the original plane of the colony and 

 occupies the position of an ordinary cell ; in some species the 

 vibracular cells alternate regularly with the z oecia. In such 

 cases the movements of the setae are of necessity much restricted, 

 and the appendage is rather the servant of the colony than of 

 the individual polypide. 



'' The direct links between the vibraculum and avicularium are 

 found in those forms of the latter in which the mandible is pro- 

 longed and attenuated, whilst the beak is almost rudimentary. 

 Indeed, it is difficult to draw the line between them, unless we 

 regard the total absence of a distinct beak as an essential charac- 

 teristic of the vibraculum. We have a case of the slight exten-. 

 sion and attenuation of the mandible in Schizoporella spinifera; 

 in Memlranipora ciliata the change is occasionally carried still 

 farther, but there is a, great variability, and the mandible is now 

 of the ordinary form and now prolonged into a vibracular pro- 

 cess. In Schizoporella vulgaris the mandible is metamorphosed 



