POLYZOA. 



609 



after the death of the general colony. In the "vibracula" (fig. 

 454, a), the place of the mandible of the avicularium is taken by a 

 bristle, or seta, which is capable of extensive movement. 



Owing to the minute size of the avicularia and vibracula and 

 their readily perishable structure, it could hardly be expected that 

 the fossil Polyzoa should exhibit similar organs except under the 

 most favourable conditions of preservation ; and as a matter of fact, 

 these appendages have not hitherto been recognised except in a few 

 instances. The former existence of avicularia may, however, be 

 in many cases inferred by the presence of cicatrices or pore-like 



Fig- 454- — A > Three cells of Mastigophora Hyndma?ini, showing vibracula (v) — the left-hand 

 cell also showsan ocecium; b, Sessile avicularium of Scrupoccllaria scruposa; and c, Pedun- 

 culate avicularium. b, Beak ; c, Chamber of the avicularium ; m, Muscles ; p, Peduncle. 

 All the figures are enlarged. (After Hincks and Busk.) 



depressions on the surface of the cells. In some instances it can 

 be shown, as, for example, in the recent Retepora, that the avicularia 

 are really attached to thickened tubes which pass deep into the 

 substance of the polyzoary, and which can therefore be readily 

 recognised in thin sections (fig. 455, a). This observation is of 

 considerable interest as affording a possible explanation of certain 

 thickened tubular structures which are found in some of the Palaeo- 

 zoic Polyzoa. The structures in question have the form of tubes 

 with thickened fibrous walls, embedded in the solid tissue of the 

 polyzoary ; and when cut across transversely in thin sections (fig. 

 455, b and c), they exhibit a minute central clear spot surrounded 

 vol. 1. 2 Q 



